Arrange one rack in the upper third and one rack in the lower third of the oven, and then preheat the oven to 450°F. Combine the fennel wedges and onions in a large bowl. Add the 2 tablespoons of the oil, 1½ teaspoons of the orange zest, and 1 teaspoon of the fennel seeds and toss to coat the vegetables. Sprinkle with salt and pepper and toss. Arrange the vegetables in a single layer on a large rimmed baking sheet. Place on the upper oven rack and roast 15 minutes. Stir the vegetables and continue roasting until tender and beginning to brown, about 15 minutes longer.

Meanwhile, spray a small, rimmed baking sheet with non-stick spray, or brush with olive oil. Brush the skin of the fish with olive oil and place skin side down on the prepared baking sheet. Brush the top of the fish with olive oil and then sprinkle with salt, pepper, 1½ teaspoons of the orange zest, and remaining ¾ teaspoon fennel seeds.

After the vegetables have cooked a total of 20 minutes, place the pan with the fish on the lower oven rack and cook the fish until it feels springy when pressed in the center, about 8-10 minutes, depending on the thickness of the fish. For another test of doneness, make a small incision in the thickest part of the fish, the flesh should be just opaque in the center; if it is still translucent, cook a few minutes longer.

While the fish cooks, mix the vinegar, orange juice, brown sugar, and remaining 1 teaspoon grated orange zest in a large skillet. Stir until the sugar dissolves.

Simmer over medium heat until thickened to a syrupy texture, stirring frequently, about 3 minutes. Remove from the heat.

Divide the vegetables and fish among 4 warmed plates, saving the juices on the fish pan. Stir 3-4 teaspoons of the fish juices into the balsamic sauce. Spoon the sauce over the fish and vegetables. Garnish with fennel fronds and serve immediately.