Behind the Recipes

Tuscany’s Forgotten Onion Soup

The buttery, sweet savoriness of carabaccia, Tuscany’s centuries-old red onion soup, introduced us to an entirely different side of allium complexity.
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Published Feb. 5, 2024.

You’ve probably never had carabaccia. Neither had I until recently, nor have most Italians even though Tuscan cooks have been making it since at least as far back as the 15th century.

The understated red onion soup is a point of pride in and around Certaldo, a hilly town just south of Florence known for its eponymous wine‑colored bulb, but for most of the world, the pleasures of its sweet savoriness and nourishing comfort have been kept under wraps.

Until now, I hope.

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The Roots of Carabaccia

Certaldo, the Tuscan town that sits just south of Florence, is famous for its medieval red bricks, steep hills, and onions. The namesake red bulb, round in the middle and flat on either end, is so tied to the local agriculture that it’s lionized in the town crest and trademarked by the Slow Food presidium.

Even the townspeople affectionately refer to themselves as cipolloni, or “big onions.” 

Besides the implicit virtues of soup (belly warming, hydrating, economical), there’s something compelling about a dish such as carabaccia that conjures depth of flavor from almost nothing.

Early versions consisted of little more than softened onions simmered in water with ground almonds (a thickener and a source of protein) and seasoned with vinegar, honey or sugar, and cinnamon—a meatless (“di magro,” or “lean”) dish that would have been common during the Christian calendar’s many religiously imposed fasting days. 

Gradually, tastes changed, though modern versions are still sparse. The soup’s sweet, sour, and warm flavors were replaced by savory ones such as Parmesan or Pecorino. Almonds fell away while bread added sustenance, as did the occasional poached egg. 

What impressed me most about the versions I tried was that gentle savoriness of the onions—a side of allium flavor I’d never known. The soup was so simple to make too.

All I did was soften the onions (common domestic red ones instead of the hyperlocal Certaldo kind). Simmer them in water with fragrant bay leaves and sage. Season the pot with grated cheese, salt, and pepper.

Ladle each portion over a slab of toast, the crispness and char of which beautifully offset the simple broth.

And—for a bit of luxury—top it with a poached egg, which spilled its velvety richness into the broth. It was easy comfort food that would keep me warm until spring. 

“...accia”

Carabaccia’s humbleness is embedded in its name—specifically in the suffix “accia,” which means “bad” or, playfully, “common.” According to food historian Karima Moyer-Nocchi, it refers to the Great Chain of Being (medieval Christianity’s hierarchal structure of all matter and life) belief that foods grown above ground were fit for an aristocrat’s digestive system and those grown below ground, such as onions and garlic, were not. “Accio,” the masculine form, carries a similarly inelegant connotation, as in the homey Neapolitan millet or semolina cake named migliaccio.  

Onion Breakdown

All of my carabaccia sources stressed that the onions should soften—but not brown.

“Browning and caramelizing, it’s just not the thing,” said Karima Moyer-Nocchi, food historian and author of books including Chewing the Fat: An Oral History of Italian Foodways from Fascism to Dolce Vita (2015), when we discussed the cooking and flavor of the onions in the soup. “Having them melt is the thing.”

A popular approach to softening, according to recipes I found, is to cook them very slowly (for up to 11/2 hours) in a little olive oil, leaving the lid off and stirring frequently at the end so that they don’t stick.

It delivered as promised: The onions broke down and tasted buttery and savory-sweet—a stop along the way to caramelization that I’d overlooked until now. 

Science: The Overlooked Flavor of Softened Onions

Onions have incredible range. The pungent bite of a raw one enlivens everything from burgers to chana masala. Caramelizing (technically, Maillard browning) transforms the bulb’s sugars and proteins, while charring takes it to a new level of tarry, bitter complexity.

But there’s a lesser-known zone of onion flavor that lives between those extremes. Cooked until they’re buttery soft but pale, onions (yellow, red, and white cultivars) turn gently savory—a subtlety that gets drowned out when onions brown but that gives a simple dish such as carabaccia unmistakable depth.

The cause of this flavor development is twofold. First, flavor precursors in the onions concentrate when the bulb’s juices evaporate, encouraging more flavor‑forming reactions among them.

Meanwhile, the onions’ sulfur compounds (such as mercaptomethyl pentanol, or MMP) that started to develop when their cells were damaged during cutting, kick into high gear when they cook, forming loads more flavor. Then, continuing to simmer the onions in the soup allows these flavors to permeate the dish.

But I had a speedier method in mind, which incidentally I borrowed from Senior Editor Lan Lam’s approach to caramelized onions. The trick—which sounds counterintuitive—is to first simmer sliced onions in a little water. That way, steam surrounds and cooks them from all sides instead of just from the bottom up and rapidly softens their pectin so that they release plenty of their own water and break down faster. 

I sliced and simmered 2 pounds of them in a covered Dutch oven with 1 cup of water plus a little olive oil and salt. They were soft in just 20 minutes—a huge time savings compared to the traditional method. Then I briefly cooked them uncovered so that the liquid would evaporate—and that’s when I noticed their flavor shift from watery to subtly but richly savory.

The whole process hadn’t taken more than half an hour.

Soup’s On

There’s no historical tradition of vegetable broth in Italian cooking, Moyer-Nocchi told me. She said water would have been standard here, so I added 4 cups along with a bay leaf and sprigs of fresh sage, and simmered the pot, covered, for 30 minutes.

The soup tasted even more complex than the onions themselves, thanks to the long simmer: Cooking the onions in liquid causes some of their components to react and form a meaty-tasting sulfur compound called mercaptomethyl pentanol (MMP). Lots of Parmesan boosted the savoriness even more.

Technique:  Three Stages of Flavor Development

This method softens the onions quickly and maximizes their savory sweetness.

1. Simmer with Lid On Cooking the onions covered in a little water traps steam.

2. Stir with Lid Off  Evaporating water leads to savory depth.

3. Simmer in Soup  Further cooking the onions in the soup builds even more meaty savor.

While the soup simmered, I broiled thick slices of bread with a little olive oil until they charred at the edges and poached the eggs.

Then I ladled the soup around the bread and rested a plump, jiggly egg on top, nicking it with the side of my spoon so that it released the golden yolk into the broth. The bread softened, turning almost custardy as it soaked up the sweet-savory broth, adding a bitter, singed complexity all its own. 

It was one of the coziest soups I’d ever had. And the fact that it came together from almost nothing made it that much more satisfying.

Recipe

Carabaccia (Tuscan Onion Soup)

The buttery, sweet savoriness of carabaccia, Tuscany's centuries-old red onion soup, introduced us to an entirely different side of allium complexity.
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