Oil-poaching or butter-poaching is an elegant way to slowly cook meatier cuts of fish and seafood . It does require a fair outlay of oil; limit the overall amount by choosing a pan in which the tuna filets fit tightly but without touching each other or the edges of the pan.
Season the tuna by sprinkling it with salt; set the tuna aside while you prepare the oil (this has the added benefit of taking the refrigerated chill off the tuna before cooking it.
Choose a medium frying pan or large saucepan big enough to hold the fish in a single layer. Add enough oil to come about 1/2 inch up the pan.
Add the garlic, peppercorns, bay leaf, orange or lemon zest, and chile (if using).
Warm the oil over medium-low heat just until bubbles form on the sides of the pan. Add the tuna; use a spatula or kitchen tongs to gently lower each filet into the oil, taking care not to drop the fish in or splatter the hot oil.
The oil should cover the tuna (if it doesn’t follow the directions in parentheses). Adjust the heat to maintain those few bubbles on the sides of the pan. Gently simmer until the tuna’s cooked through, 8 to 10 minutes for a 1 inch-thick piece of tuna (if the oil doesn’t quite cover your filets, you may want to flip the fish over at the half-way point. While the fish cooks, set a cooling rack over a rimmed baking sheet or layers of papers towels; or simply line a plate of a few layers of paper towels.
Use a spatula or slotted spoon to lift the tuna out of the oil and onto the prepared cooling rack or towels to drain. Serve the filets whole for diners to cut up themselves; slice them to serve; or, flake the filets into something that resembles canned tuna.
