Place oven rack in center of oven. Preheat broiler to high. In a small skillet over medium-high heat, stir together orange juice, brown sugar and garlic. Bring to a boil and cook until thickened and glossy, 4-5 minutes. Cool to room temperature; stir in sauce.
Arrange fish on a broiling pan on a rack over a rimmed baking sheet. Sprinkle with salt and pepper. Brush liberally with some of the sauce. Broil, basting twice with sauce, until fish flakes easily with a fork, 8-10 minutes. Garnish with green onions.