Heat oven to 375°F. In 12-inch skillet, cook beef and onion over medium heat 8 to 10 minutes, stirring occasionally, until thoroughly cooked; drain. Set 12 peas (from frozen vegetables) aside for garnish. Add remaining frozen vegetables, tomatoes and gravy to beef mixture. Heat to boiling. Reduce heat to medium-low. Cover and cook 8 to 10 minutes, stirring occasionally, until vegetables are crisp-tender.