You'll end up with extra confit, but it keeps for at least 10 days, and is great on salads and sandwiches.
1 pound skinless yellowfin (ahi) or albacore tuna fillet (1 in. thick), in one piece
2 teaspoons kosher salt
3/4 teaspoon dried oregano
3/4 teaspoon freshly ground white peppercorns
3/4 teaspoon freshly ground fennel seeds
1/4 teaspoon red chile flakes
1 bay leaf
2 sprigs each flat-leaf parsley and fresh mint
2 small garlic cloves, peeled and crushed
3 wide strips lemon zest
3 cups extra-virgin olive oil
16 baguette slices
Minced flat-leaf parsley, radishes, and green olives, for garnish
How to Make It
Cut tuna in half lengthwise. Combine spices in a small bowl. Season tuna all over with spices and put in a 4- by 8-in. loaf pan. Chill, covered, 3 to 5 hours.
Preheat oven to 300°. Add bay leaf, herb sprigs, garlic, and zest to tuna, tucking them around fish; pour in enough oil to cover by 1/4 in.
Bake tuna 25 minutes, uncovered, lowering heat if necessary to keep oil from boiling. Let tuna cool to room temperature in oil.
Lift one piece of tuna from oil; separate into thick flakes. Top baguette slices with tuna, using more if needed, and arrange on a platter with olives and radishes. Scatter minced parsley over platter. Drizzle tuna with some of the oil, and flick some oil onto the olives and radishes too.
Make ahead: Through step 3, up to 10 days, chilled. Bring to room temperature before serving.