Set a large heavy skillet over medium-high heat and add oil. Add beef and stir-fry until no more pink shows. Stir in garlic for a few seconds, then add the mushrooms and their liquid, the onion soup mix, water and tomato paste. Mix well, bring to a full boil, cover,and remove skillet off heat.
In a cup, stir together sour cream and flour until lump-free. Stir into beef mixture and return to heat. Warm through well, but do not boil. Add salt and pepper to taste and serve over hot buttered noodles or mashed potatoes.