In a small bowl stir together Parmesan cheese, bread crumbs, and the 2 teaspoons melted butter; set aside.
Separate caps and stems from shiitake and oyster mushrooms. (Reserve stems to use in stocks or discard.) Slice mushroom caps.
In a large skillet cook button mushrooms and garlic in the 2 tablespoons butter over medium-high heat about 5 minutes or until tender and most of the liquid has evaporated, stirring occasionally. Remove mushrooms and set aside, reserving drippings in skillet. Add the shiitake and oyster mushrooms to the skillet. Cook for 7 to 8 minutes or until tender and most of the liquid has evaporated, stirring occasionally. Stir in the flour, mustard, thyme, and salt. Add milk all at once. Cook and stir until thickened and bubbly. Cook and stir for 1 minute more. Stir in the button mushroom mixture.
Transfer mushroom mixture to a 1-quart au gratin dish or 1-quart casserole. Sprinkle with the bread crumb mixture. Bake, uncovered, in a 350 degree F oven about 15 minutes or until heated through.