Cook meat in a large skillet, breaking it up with a spoon as it cooks, until no pink remains. Drain. Stir in onion; cook until tender, about 3 minutes. Blend in broth, mustard and tomato paste. Bring to boil; lower heat and cover and simmer 5 minutes. Stir in sour cream very slowly to prevent curdling. Continue to heat gently, but do not allow to boil. Serve over hot cooked noodles.
Please note that some ingredients and brands may not be available in every store.