- 3 tablespoons olive oil
- 1 large onion, halved lengthwise, then thinly sliced crosswise
- 3 tablespoons water
- 1/2 teaspoon black pepper
- 3 tablespoons unsalted butter
- 3 tablespoons capers (in brine), rinsed, drained, and coarsely chopped
- 2 1/2 tablespoons fresh lemon juice
- 4 (6-oz) pieces mahimahi fillet (1 1/2 to 2 inches thick) with skin
- 3/4 teaspoon salt
- 2 tablespoons chopped fresh flat-leaf parsley
- Heat 2 tablespoons oil in a 12-inch heavy skillet over moderately high heat until hot but not smoking, then sauté onion, stirring frequently, until golden, about 6 minutes. Stir in water and 1/4 teaspoon pepper and cook, stirring, until onion is softened, about 1 minute. Stir in butter until melted, then stir in capers and lemon juice. Remove from heat and keep warm, covered.
- Preheat broiler.
- Pat fish dry, then brush all over with remaining tablespoon oil and sprinkle with salt and remaining 1/4 teaspoon pepper. Put fillets, skin sides down, on rack of a broiler pan and broil about 5 inches from heat until just cooked through, 6 to 8 minutes.
- Serve fish topped with onion mixture and sprinkled with parsley.