A few years ago, before sous vide infiltrated restaurant kitchens, every other menu featured a "slow-cooked" fish or shellfish. Slow cooking may sound like a daunting technique, but it's really just what it advertises -- fish or shellfish cooked gently and patiently over low heat, usually in a warm bath of olive oil or butter.
Slow poaching is now the technique I use most often on weeknights when I'm busy and want a low maintenance but delicious main course, and for dinner parties, when I want to serve fish but don't want to worry about sauteeing at the last minute.
We have a terrific fish vendor at our local farmers market (Borough Hall in Brooklyn Heights). His seafood is so fresh and pristine that there's nothing more I want to do with it than cook it simply, seasoned with sea salt and spritzed with a lemon wedge at the table. And I certainly don't want to risk overcooking it, which is one of the reasons poaching fish or shellfish in olive oil is such a genius method. An oil bath creates a protective cocoon around the seafood so none of the edges dry out, and just enough of the oil clings to it to give the seafood a buttery feel. And because of the low temperature, the seafood isn't done one minute and overcooked the next -- day-dreamers and multi-taskers get a time buffer. Chances are you'll end up with fish or shrimp that tastes pure and clean, and pairing possibilities that are endless (think salsa verde, romesco, grits and risotto).
To poach any kind of fish: Lay the fillets in a shallow, oven-proof casserole dish or skillet, just large enough to hold the fish in a single layer. Cover the fish with a 1/8-inch thick layer of olive oil (a good brand, but not your best), season with a flakey sea salt and any other herb or spice you like, then send it into a 275-degree oven, basting it often, until it's cooked through. For a 1-inch thick fish fillet, it takes about 30 minutes.
To poach shrimp, scallops or lobster: I like to cook them on the stovetop. Place them in a single layer in a saucepan and pour in enough oil to just cover them. My default aromatics are thyme and lightly smashed garlic cloves (see photo above). Then set the pan over low heat, letting it warm enough so that tiny bubbles begin emerging on the sides of the pan, but none of the shellfish are bouncing around. Baste often and you'll see the shellfish slowly turn opaque and constrict. When they're cooked properly, they'll be bouncy and light and not at all tough.