Vegetable-Stuffed Rainbow Trout in a Casserole

Cut the rainbow trout into large slices. Place some of the onions, sliced into half-moons, in the casserole dish. Arrange the fish pieces on top, then add the vegetables. Add salt and drizzle with olive oil. Cover the casserole dish and bake in a 200°C oven for 30 minutes. Then uncover and continue baking. Add the butter and bake for another 10 minutes. Serve hot.