Preheat oven to 400°F.

Arrange the fish in a shallow baking dish. In a small skillet sauté the onion and the garlic until the onion is tender but not brown. Into the pan, stir in the cilantro, salt and pepper. Spread the mixture over the fish. Combine the orange juice and lemon juice and pour evenly over the fish. Bake, uncovered, for about 20 to 25 minutes, or until the fish flakes easily with a fork.