Melt butter in skillet; add mushrooms & onions & saute until tender. Place mushrooms & onions in shallow, 2-quart casserole & set aside. Combine flour, salt, paprika, & pepper; dredge venison in flour mixture. Melt butter in skillet, add venison, & brown well. Place venison over onions & mushrooms in casserole. Top with garlic & rosemary. Pour beef stock over top. Loosen browned particles from skillet & add to casserole. Cover & bake 350 degrees Fahrenheit for 45 minutes. Serve sauce in gravy boat.