I love this dish made with the whole fish, but fillets are much more practical. Halibut is a firm, meaty fish which holds together well when baked with vegetables. On the other hand, you can replace it with your favorite fish fillets (just keep in mind the cooking time will vary with the fish you choose). This dish is best made with porcini mushrooms, but assorted other mushrooms, such as portobello or shiitaki, will also yield good results. If you do not like mushrooms, sliced zucchini is a delicious option.
In cooking fish, especially in the oven, it is important to use the oven heat to your advantage, so you don’t overcook the fish but can simultaneously cook the sauce and the vegetables. In this recipe, I tell you to put the fish on the bottom of the oven, because the heat coming from the bottom is greater and will cook the sauce and vegetables faster and not overcook the fish. Also keep in mind that if you want a crispy top you must put the fish or whatever you are cooking on the top shelf of the oven. When you cook on the middle shelf in the oven, the cooking temperature circulates and is distributed evenly around what you are cooking. Shift food around in the oven according to the end results you want to achieve.
4 medium porcini or portobello mushrooms
7 tablespoons -extra--virgin olive oil
3 medium onions, thinly sliced
6 garlic cloves, peeled and crushed
1 teaspoon kosher salt
6 skinless halibut fillets, about 2¼ pounds total
Juice of 1 lemon
½ cup white wine
2 tablespoons chopped fresh parsley
Preheat the oven to 400 degrees F. Slice the mushrooms on the bias in 1-½--inch--thick slices. Remove the stems from the mushrooms, and shave off the gills if using portobellos. In a large skillet, heat 3 tablespoons of the olive oil over medium heat. Add the onions and garlic, and cook until wilted, about 4 minutes. Spread the onions and garlic on an oiled half sheet pan.
In the same skillet, over -medium--high heat, heat 2 tablespoons of the olive oil, and add the mushroom slices (or do in batches if the skillet is too crowded). Drizzle with 2 tablespoons of water. Cook and flip the mushrooms until they are softened and browned, about 4 minutes (per batch, if you’re doing it in batches). Spread half of the mushrooms on top of the wilted onions, and season all with ½ teaspoon of salt.
Season the halibut with half of the lemon juice, the remaining ½ teaspoon of salt, and the remaining 2 tablespoons of olive oil. Pour the remaining lemon juice and the white wine over the vegetables, and sprinkle with the parsley. Set the halibut on top, and top each fillet with remaining mushroom slices. Bake the fish on the bottom rack of the oven until it is cooked through and the juices are reduced and saucy, about 18 to 20 minutes.