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.