Place ground chuck and onion in a 1 1/2-quart casserole; cover.
Microwave 2 1/2 minutes; stir.
Cover; microwave 2 1/2 minutes.
Pour off grease and stir in mustard, tomato sauce, soy sauce, garlic powder, pepper, horse radish and bean sprouts.
Microwave 2 minutes.
Stir in sour cream.
Microwave 30 seconds.
Spoon on buns.