Preheat broiler and oil a shallow baking pan large enough to hold fillets in one layer.
In a dry small skillet toast fennel seeds over moderate heat, stirring, until fragrant. Mince fennel seeds and garlic with salt and in a small bowl stir together with mayonnaise, lemon juice, and pepper to taste.
Arrange fillets in pan and spread fennel mayonnaise evenly over tops. Broil fillets 3 inches from heat until just cooked through, about 7 minutes.
