By Published Aug. 29, 2023.
This is Technique #64 from our 100 Techniques Every Home Cook Can Master.
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On paper, cooking delicate fish fillets in lots of oil sounds like a recipe for greasy disaster. But the stunningly moist and tender results explain why this technique is so popular in high-end restaurants and serve as a reminder of why poaching became a classic approach to cooking fish in the first place.
Poaching fish—submerging it in liquid and gently cooking it at a below-simmer temperature between 130 and 180 degrees—renders the delicate flesh silky and supple. With this particular technique, though, rather than the usual lean bath of water, wine, or broth, we use olive oil.
Contrary to what you might assume, fish poached this way actually absorbs very little oil. In order for the oil to penetrate the fish, moisture must exit first. But because oil and water repel each other, it’s very difficult for the water inside the fish to exit.
Hence, more of the moisture stays in the fish. We learned in testing that while water-poached fillets lost 24 percent of their weight during cooking, oil-poached fish lost just 14 percent—leading to remarkably moist, velvety results.
You don't even need to submerge the fish entirely in the oil. Using a small pan, displacing some of the oil with a halved onion, and flipping the fillets partway through collectively allow you to poach four fillets in just 3⁄4 cup oil.
One of the niftiest things we learned is that the fish cooks more gently and slowly in oil than in water—even when both liquids are exactly the same temperature. This has to do with oil’s specific heat capacity, or how much energy is needed to change its temperature by 1 degree Celsius.
A key to success is keeping the oil at a steady temperature to slowly and evenly bring the fish to the ideal internal temperature of 130 to 135 degrees. To avoid temperature spikes in the small amount of oil without having to fiddle constantly with burner knobs, heat the oil on the stovetop to well above the target temperature, then nestle in the fillets (which lower the oil’s temperature), and then rely on the oven’s even heat to keep the oil in the poaching sweet spot.
Oil has about half the heat capacity of water, which means it requires half the amount of energy to reach the same temperature as an equal volume of water. As a result, it has less energy to transfer to food and thus will cook the food more slowly than water.
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While you might be hesitant to dunk your fish fillets in a pan of olive oil, this method is backed by science. Not only will the fish remain more moist than if it were cooked in water, but it will cook more gently. Check out our step-by-step guide below to learn how to do this in your kitchen.
Pat fish fillets dry, sprinkle with salt, and let rest at room temperature.
Bring oil in 10-inch skillet to 180 degrees and carefully place onion half in center of skillet to displace some oil.
Arrange fish fillets, skinned side up, around onion (oil should come roughly halfway up sides of fillets). Spoon a little oil over each fillet, cover skillet, and cook in 250-degree oven for 15 minutes.
Using 2 spatulas, carefully flip fillets. Cover skillet, return to oven, and cook until fish registers 130 to 135 degrees.
Transfer fish to serving platter and use some of flavor-infused cooking oil to blend into sauce.
Want to test out your newfound knowledge of seasoning? Try it with any of these recipes.
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