Bring a large pot of water to a boil. Cook noodles until tender, about 8 minutes. Drain.
In large skillet, cook mushrooms and onions in 2 Tbsp butter until soft; remove from pan.
Melt remaining butter in same pan. Cook ground beef in butter until browned. Mix in flour. Stir in beef broth, and cook all until slightly thickened. Stir in mushroom and onion mixture and sour cream. Season to taste with salt and pepper.
Continue cooking until sauce is hot, but not boiling
Serve sauce over egg noodles.