In an oven-proof baking dish, place slices of onion on the bottom, then slices of butter on top of the onion. Next, place halibut chunks about 1 inch thick on top of the butter. Spread a heavy coat of mayonnaise on the halibut and top with slices of butter. Sprinkle dill weed and paprika on the mayonnaise and butter. Bake, uncovered, for 15 minutes at 350 degrees. Take it out and let it sit for 5 minutes to finish cooking.