The firm flesh of halibut makes it the perfect choice for this dish, but feel free to substitute salmon, cod or any other thick fish. The bell peppers can be varied as well—exchange green for red or use a combination of red, green and yellow. You can also add rosemary, basil or even mint to the gremolata, a classic Italian seasoning of parsley, garlic and lemon zest. Accompany the dish with greens, such as spinach or chard.
- Preheat oven to 400°F. Place potatoes in a large roasting pan or on a large rimmed baking sheet; drizzle with oil and turn to coat evenly. Add bell pepper and onion. Season with 1/4 teaspoon salt and pepper.
- Roast the vegetables, turning the potatoes once or twice and moving the pepper and onion pieces around so they brown evenly, until the potatoes are starting to brown and are almost tender, about 35 minutes.
- While the vegetables are roasting, finely chop parsley, lemon zest, oregano and garlic together to make gremolata. Season halibut with remaining 1/4 teaspoon salt and pepper, then sprinkle with 2 teaspoons gremolata.
- Remove the pan from the oven. Increase oven temperature to 450°. Push the vegetables to the sides of the pan and place the halibut in the center. Spoon some of the onions and peppers over the halibut. Arrange the potatoes around the edges, turning the browned sides up.
- Roast until the vegetables are browned and tender and the halibut is opaque in the center, 10 to 15 minutes more, depending on the thickness of the fish. Sprinkle the remaining gremolata on top. Arrange the halibut and vegetables on a platter or individual plates. Serve with lemon wedges.