Preheat the oven to 400°. In a bowl, mix half of the lime zest, lime juice, garlic and chile. Stir in the olive oil. Make 3 shallow 2-inch slashes into the skin of each fish fillet. Spread the garlic-chile paste over the fish. Transfer the fish to a baking dish, skin side up; season with salt and pepper. Bake for 8 minutes, until the fish flakes easily.
Meanwhile, in another small bowl, combine the mayonnaise with the capers and the remaining lime zest, lime juice, garlic and chile. Season the sauce with salt and pepper.
Toast the buns. Spread the sauce on the buns and top with the fish, lettuce and onion. Close the sandwiches and serve.
One serving 441 cal, 20 gm total fat, 3.2 gm saturated fat, 68 gm carb, 2 gm fiber.