Combine ground beef, bread crumbs, milk, garlic and egg in a large bowl. Shape into 4 oval patties.
Melt 1 Tbsp. margarine in a large nonstick skillet over medium-high heat and brown patties, turning once. Remove patties and keep warm.
Melt remaining 1 Tbsp. margarine in same skillet over medium-high heat and cook mushrooms, stirring frequently, until lightly browned, about 3 minutes. Stir in water and Knorr® Classic Brown gravy mix and bring to a boil. Return patties to skillet. Reduce heat and simmer until patties are done, about 5 minutes. Stir in sour cream and heat through.
Serve, if desired, with hot cooked noodles or mashed potatoes.