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. Add frozen vegetables, tomatoes, thawed spinach, gravy, salt and 1/4 teaspoon garlic powder 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.