When I worked the fish station at Troquet, a now-shuttered bistro that overlooked Boston Common, I prepared anywhere from four to six different dishes during each service. Most involved a smoking-hot pan for crisping skin-on striped bass fillets or searing scallops, but every so often my chef would run a dish that offered a reprieve from all that sizzling heat.
How the French Poach Fish
Published Apr. 3, 2023.
French Technique, Flexible Flavor
You can prepare fish à la nage by infusing the poaching-turned–serving liquid with classic French flavors like wine, leek, lemon, and bay leaves. Or you can do as I do and think of this preparation as a template for incorporating all sorts of seasonings. Taking influence from Southeast Asia, I pair sweet-spicy lemongrass and ginger with scallion, dill, fresh chile heat, and lime juice. And instead of water and wine, I use subtly sweet coconut water spiked with umami-rich fish sauce.
It was à la nage, French for “in the swim” and for the classic method of gently poaching and serving fish in a fragrant broth. To make it, I’d slip a fillet of something flaky and white like halibut or cod into a pan filled with gently simmering broth. It only took minutes for the fish to cook through and turn silky, at which point I’d gently usher it from the pan into a shallow serving bowl. Then I’d ladle a light broth over it for serving—either the poaching liquid that I’d strained and spruced up with simple seasonings or a fish-based stock that I had at the ready; embellish the bowl with whatever colorful, high-flavor garnishes were at my station; and send it off to the dining room.
It looked polished and admittedly cheffy, but its panache belied the ease of making it. Plus, any pro cook will tell you that poaching is one of the smartest ways to cook delicate fish. It’s quick, simple, and gentle, and the clear, light cooking and serving liquids don’t obscure its mellow flavor the way searing, frying, or rich sauces do. The beauty of à la nage in particular is that even though the OG is French through and through, there are countless ways to vary the flavors.
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Science: Cut (the Heat) and Cover
White fish is fully cooked at 135 degrees, but you should pull it off the heat at 110 and place a lid over the pan. The amount of steamy liquid, along with the cover, will hold enough heat to carry the fish across the finish line within a few minutes—and will do so more gently than active cooking could, so there’s no virtually no risk of overdoing it.
One benefit to preparing this dish in a restaurant was that I had a reserve of stocks, sauces, and garnishes for adding those critical points of nuanced complexity and color. Pulling it off at home meant capturing everything those elements offered in the simplest way possible, and the most obvious place to start would be designing one all-purpose liquid for poaching and serving.
Typically, the fish cooks in court bouillon (literally “short broth”), a combination of water and an acidic liquid like wine, lemon juice, or vinegar (acid reduces the volatility of fish’s unpleasant aromatic compounds, minimizing fishiness) that’s infused with chopped aromatic vegetables and fresh herbs. The “nage” itself, as cooks refer to the serving broth, is less prescriptive but should be medium-bodied, nicely acidic with a touch of richness, and lively with herbs.
I had the matchup of lemongrass and dill stuck in my head from a seafood dish I’d recently enjoyed at a Vietnamese restaurant, so I trimmed and smashed a few of the citrusy stalks—the best way to extract their aroma—along with a chunk of ginger and a few scallion whites. Then, instead of simmering those aromatics in plain water, I used a couple cups of the coconut kind and spiked it with fish sauce, lime juice, a few Fresno chile rings, and a bit of salt. (The dill would come later.)
Delicate white fish like halibut cooks through so quickly that I knew there wouldn’t be much time to concentrate the cooking liquid into something complex and cohesive enough for serving. So I simmered it without the fish for about 20 minutes, by which time the coconut water’s soft, mineral-y sweetness had melded with the acid; umami; salt; and subtle, fruity heat. It was tasty enough to sip by the spoonful, and it would be even better after taking on savoriness from the fish’s juices.
Tips for Pretty Presentation
With a few simple tricks, this dish can achieve restaurant‑caliber polish.
Mend any cracks in the raw fish with toothpicks. They will seal as the fish cooks. Remove the picks before serving.
Place the raw fillet skinned side down and perfectly flat in the pan so that it sets evenly as it cooks.
Serve in a wide, shallow bowl so that there will be space around the fillet for garnishing.
Serve skinned side down. This will display the fillet’s pure white color without any blemishes.
Stir the green oil before using to recombine any bits of herb that have settled to the bottom. Drizzle tableside so that the droplets don’t have time to bleed into the broth.
How quickly the fish cooks also makes it tricky to avoid overshooting the mark, which is 135 to 140 degrees for flaky white varieties. Even when you’re vigilant, all the heat held in the poaching liquid transfers to the fish in a flash. But here’s the cool part: You can turn that liability into a perk by cutting the heat well before the fish is cooked through (110 degrees), throwing a lid on the pan for a few more minutes, and letting carryover cooking pull it gradually across the finish line. That way, there’s virtually no risk of overcooking.
Making It Easy to Be Green
Back to the dill: I waited to add a few sprigs to the pan with the fish so that its bright aromatic compounds wouldn’t be driven off as the broth simmered, along with some lime zest to ratchet up the floral freshness of the lemongrass. Then, I used more to make a green oil (kitchen speak for verdant herb oil) for anointing the nage with richness, freshness, and pops of color.
Science: Make Vividly Green Herb Oil
Green oils add pops of vibrancy and freshness when drizzled over poached fish, soups, roasted vegetables, or dips. They can even be turned into verdant dressings by whisking the oil with lemon juice or vinegar and a dash of salt. The key to their vibrant hue is blanching and shocking the herbs: Blanching collapses the plants’ cells, intensifiying their green color, and partially deactivates the enzyme that causes oxidative browning; shocking stops the cooking process to keep them tasting fresh.
Green oils often start with parsley because its neutral, grassy freshness and abundant chlorophyll add a lot of vibrancy. The key is to blanch the leaves, which tenderizes them and intensifies their color, so I tossed them in a pot of salted water with the scallion greens I had leftover from the nage for about 30 seconds and shocked the herbs in cold water so the heat wouldn’t destroy their pigments. After squeezing them dry, I buzzed them with dill and vegetable oil until the mixture was bright green.
Dotting the broth with the oil around the pristine, silky fish added just enough richness and polish; putting it together took me back to my Troquet days and inspired me to create a Western European variation with parsnips, tarragon, whole-grain mustard, and Parmesan broth.