Cut meat across the grain unto ¾” slices, then into strips 3x¼”.
Melt ¼ cup butter in large skillet or pot over medium heat. Cook mushrooms in butter 8-10 min or until browned. Remove mushrooms with slotted spoon and set aside, leaving remaining butter in pan.
In same skillet, brown meat. Reserving ⅔ cup of broth, stir in remaining broth, ketchup, and garlic salt. Cover and simmer 15 min. Start water to boil for noodles, cook when ready.
Blend reserved broth and flour; stir into meat. Add mushrooms. Heat to boiling, stirring constantly. Boil and stir for 1 minute. Stir in sour cream; heat through.
Toss noodles with 3 tbsp butter. Serve with stroganoff.
