Melt butter; pour into bottom of 13X9-inch baking pan.
Spread onion over melted butter and top with halibut fillets.
Mix together sour cream and mayonnaise; spread over halibut.
Top with cracker crumbs.
Bake in oven until fish flakes and crumbs are lightly browned (about 20 minutes.).
*Note* If you can’t get your hands on fresh halibut, I’ve learned to cut all the brown parts off the halibut and then soak it in about 1/2 gallon of water with 1/4 cup lemon juice for about an hour. This usually takes out the fishy taste.