Olive Oil-Poached Tuna With Garlic Aioli 

Preparation

1. Step 1

   For the fish: Pat the fish dry with paper towels, season all over with salt and set aside for 10 to 15 minutes so the salt can absorb.

2. Step 2

   While the fish is resting, using a vegetable peeler, remove wide strips of zest from the lemon. Add the strips of zest to a medium saucepan (just large enough to hold the fish) along with the onions, garlic, thyme, chile and olive oil. Set over medium-low heat and let the oil slowly warm and infuse until the onions begin to lose their redness and soften, 12 to 15 minutes. Hold onto the zested lemon for the aioli and for serving later.

3. Step 3

   Reduce the heat to low. Using tongs, gently slip the fish into the oil. If it’s not fully submerged, add just enough oil to cover. Let the fish poach until opaque and tender, 15 to 20 minutes, depending on thickness. You will see the occasional bubble come up to the surface, but there should not be any rapid bubbles. The oil should very gently cook, or confit, the fish.

4. Step 4

   Use a slotted spoon or spatula to carefully transfer the fish to a plate; it should flake easily but still hold its shape. Scoop out some of the softened onions and chile to serve with the fish and reserve the garlic cloves for the aioli in a small bowl. Strain the leftover confit oil through a fine-mesh sieve, reserving ½ cup for the aioli. (The confit oil will keep, refrigerated, for up to 5 days.)

5. Step 5

   For the aioli: In a medium bowl, whisk the egg yolk and mustard until glossy. Begin adding the ½ cup leftover confit oil, a few drops at a time, whisking constantly to create an emulsion. Once it starts to thicken, gradually stream in the rest of the ½ cup confit oil and then the neutral oil. When thickened, add the reserved garlic cloves and mash up using a fork. Cut the reserved lemon in half and squeeze the juice from one half into the aioli. Season with salt and stir.

6. Step 6

   Spoon some of the softened onions and chile over the fish, add a generous dollop of the aioli, and finish with flaky salt and freshly ground pepper, and sprinkle of fresh herbs, if using. Cut the remaining half of the lemon into wedges and serve on the side.