If necessary, scale and prepare for cooking:
Preheat the oven to 350°F. Butter a shallow baking dish.
Combine in a medium saucepan:
Simmer until tender but not mushy, 15 to 20 minutes. For the last 5 minutes of cooking, add:
Drain well and transfer to a bowl. Pat the fish dry and place in the prepared baking dish. Toss the potatoes and onions with:
Arrange the vegetables around the fish. Brush the fish with:
Sprinkle with:
Bake for 20 to 30 minutes, depending on the thickness of the fish. The flesh should be firm and opaque throughout, or reach an internal temperature of 125° to 130°F, the latter temperature for flakier fish (for more on how to test for doneness, see here). Serve in the baking dish; for serving whole fish, see here. Garnish with: