Partially freeze beef; thinly slice across grain into bite-size strips.
In wok melt butter over medium-high heat.
Add half the beef and stir-fry till browned; remove.
Repeat with remaining beef.
Stir-fry onions and green pepper till tender.
Return beef to wok.
Stir in undrained tomatoes, broth, paprika, and 1 teaspoon salt.
Cover and simmer 10 to 15 minutes or till meat is tender.
Blend together sour cream and flour; gradually stir in about 1 cup hot mixture.
Return all to wok.
Cook and stir till thickened and bubbly; cook and stir 1 minute longer.
Serve over noodles.