Remove stems from mushrooms; chop stems. Set mushroom caps aside.
Melt butter in a heavy skillet. Add chopped stems, veal, onion, and garlic; sauté mixture until veal is browned, stirring to crumble. Add seasonings; mix well. Drain off pan drippings.
Spoon mixture into mushroom caps; place caps in a lightly greased 13- x 9- x 2-inch baking dish. Bake at 375° for 20 minutes. Spoon sauce over mushrooms to serve.