Reduce heat to medium; melt remaining 1 tablespoon butter in skillet. Add onion and meatballs; cook 4 to 6 minutes, stirring occasionally, until onions are tender and meatballs are heated through. Add broth, tomatoes and gnocchi to skillet; heat to boiling. Cover; reduce heat to medium-low. Simmer 3 to 5 minutes or until gnocchi is tender; turn off heat.