Step 1Trim any skin, bones or dark blood spots from the tuna and discard. Cut the tuna into large cubes (at least 1 inch) and place them in a 1-quart saucepan. Add the salt, red pepper flakes, garlic and a pinch of black pepper and stir gently to distribute seasonings evenly. Arrange the tuna snugly in the pan so you won't need to use too much additional oil. Add the bay leaf and lemon peel and pour over enough olive oil to just barely cover the fish. It will be about 1 cup, though you may need a little more for topping off.
Step 2Place the saucepan over very low heat and cook until the tuna just begins to flake, about 15 minutes. The oil may get hot enough that a few bubbles rise from the bottom, but it should not simmer. The top temperature shouldn't exceed 160 degrees.
Step 3Cool the tuna to warm room temperature in the oil before transferring to a container for storage in the refrigerator. If you're going to use the tuna the same day, refrigeration is not necessary. The tuna will keep, tightly sealed and refrigerated, for at least a week, but not more than 10 days. Warm to room temperature before using.