Similar to tuna, yellowtail is most often grilled or pan-seared, and served quite rare. Thanks to yellowtail's rich, oily texture, however, it needn't be limited to rare preparations to remain enjoyable. When oven-poached, yellowtail retains its moisture and protein despite being cooked through. Oven-poaching employs low, slow heat through the poaching liquid, sealing in the fish's moisture and nutrients that are otherwise lost to the high heat of a broiler or hot oven. While oven-poaching requires some patience, it is relatively easy to execute and offers a refreshing alternative to traditional methods of preparing yellowtail.
Preheat your oven to 250 degrees Fahrenheit. Keep the fish refrigerated until you are ready to cook. Dry the steaks with paper towels, then season them liberally with salt and pepper.
Thinly slice enough lemon to cover the bottom of a glass baking dish. If you like, crush a few garlic cloves according to taste.
Place the seasoned yellowtail steaks on the lemons in the baking dish. Cover the steaks with capers, if desired.
Pour a cooking liquid, like equal parts warm water and white wine or broth, into the baking dish until the steaks are just submerged. Add crushed garlic to the liquid, if desired.
Place the baking dish, uncovered, on a rack in the middle of the oven. Cook the yellowtail until it's just cooked through, about 1 hour.
Remove the steaks from the poaching liquid and serve immediately. If you wish, use a slotted spoon to remove some of the lemon slices and capers from the oil, and serve them with the fish.
The poaching liquid need not go to waste after you've cooked your fish. Use some or all of it to make a healthy, broth-like sauce for the fish. Simply transfer the poaching liquid to a saucepan and bring it to a boil. Reduce the heat to medium-low, and simmer the liquid until it reduces by half, about 10 minutes. Pour the broth into large, flat bowls, and place the fish in the center, garnished with the poached capers and lemons.