Butter a casserole dish. Place halibut fillets or steaks in dish and dab the fish with butter pats.
Season to taste (salt, lemon pepper, garlic powder, and dill).
Mix mayonnaise (2 cups), sour cream (1 cup), and dry mustard (1-1/2 Tbsp); slather over halibut.
Don't be stingy with the mayo mixture; liberally coating all the pieces of fish will keep them tender (and it bakes into a yummy sauce).
Sprinkle chopped green onions, sliced fresh mushrooms, and grated cheese over entire dish.
Cover and bake at 350, about 35–45 minutes (depending on thickness of fillets). Use a fork to check the halibut . . . you want the meat tender and flaky, not rubbery (overcooked) or translucent (undercooked).
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