Combine beef, oats, and bread crumbs.
Shape into 12 balls.
Arrange on microwave roasting rack placed in a 12- x 8-inch baking dish.
Microwave at HIGH 6 minutes.
Rotate dish after 3 minutes.
Remove drippings from dish.
Place cooked meatballs directly in dish.
Spread onion slices over meatballs.
Stir salt into tomatoes and spread over onions and meatballs.
Cover with waxed paper.
Microwave at MEDIUM 15 to 16 minutes.
Stir after 8 minutes.
Let stand, covered, 3 to 5 minutes before serving.