Cut meat across grain into 3/4 inch slices then into strips 3/4x1/4 inch. Melt 1/3 cup butter in skillet. Cook mushrooms ahead 5 minutes.
brown meat. Stir in broth (saving 2/3 cup) onion, catsup and garlic salt. Cover and simmer 15 minutes. Blend reserved broth & flour. Stir into meat. Add mushrooms. Heat to boiling. Boil 7 stir 1 minute. Stir in sour cream, heat through. Cook noodles, drain & toss with 3 Tbsp butter. Serve with stroganoff.