Nutrition per serving may change if servings are adjusted.
2 pounds fresh or frozen striped bass or other mild white fish fillets (skin on)
3 tablespoons olive oil, divided
3 cloves garlic, minced
1½ shallots, diced, divided in thirds
1½ teaspoons fennel seeds, toasted and chopped
2 stalks celery, bias-cut into ¼-inch-thick slices
1½ teaspoons chopped, rinsed, and drained capers
2 tablespoons white wine
1½ pounds mixed heirloom tomatoes, chopped
½ cup packed torn fresh basil leaves
1½ tablespoons roasted garlic oil
1½ teaspoons red-wine vinegar
¼ teaspoon sea salt
¼ teaspoon freshly ground pepper
1-2 bunches fresh thyme stems
Thaw fish, if frozen. Rinse the fish; pat dry with paper towels. Set aside. Finely shred the peel from the lemons; juice the lemons. Set the peel and juice aside.
For tomato relish, heat 1 tablespoon olive oil in a large skillet over medium heat. Add garlic, one-third of the diced shallot, and the fennel seeds; cook for about 2 minutes or until the shallot is translucent and the mixture is aromatic. Add celery and capers; cook for 2 minutes more. Add wine; cook over high heat until rapidly boiling. Boil until about half of the liquid has evaporated. Reduce heat to medium. Add tomatoes, basil, garlic oil, vinegar, the remaining diced shallot, one-third of the lemon peel, and one-third of the lemon juice. Cover and keep warm.
Meanwhile, preheat oven to 350°F. Season the fish with the remaining lemon peel and the remaining lemon juice. Sprinkle with sea salt and pepper. Heat the remaining 2 tablespoons olive oil in a large cast-iron skillet over medium-high heat. Add the fish, skin sides down, to the hot skillet. Cook for 45 seconds; using a metal spatula, giving each fillet a push to loosen the skin from the skillet. Add thyme stems to the oil; spoon the oil over the fillets.
Once the skin of the fillets begins to turn golden, place the skillet in the oven; bake for 6 to 8 minutes or until the fish flakes easily when tested with a fork. Carefully transfer the fillets to a paper-towel-lined plate. Let stand for 5 minutes before serving. Serve with the tomato relish.