Wipe scallops with damp paper towel. Mash garlic cloves, add to butter, and stir well to blend. Keep warm.
Pour a little of the melted garlic sauce into the bottom of a baking dish, add the mushrooms and season with onion salt and black pepper. Place the scallops on top of the mushrooms. Reserve 1 tablespoon garlic sauce and drizzle the rest on scallops.
Sprinkle with bread crumbs, parsley and reserved garlic sauce.
Bake in preheated 375°F oven until the top is nicely browned and bubbly hot, for about 10 minutes.