Spray 4- to 5-quart slow cooker with cooking spray. Sprinkle both sides of beef with flour, salt and pepper. In 12-inch skillet, heat oil over medium heat until hot. Cook beef in oil 6 to 8 minutes, turning occasionally, until brown on both sides.
Place onions in slow cooker. Top with beef and tomatoes; sprinkle with sugar.
Cover; cook on Low heat setting 7 to 9 hours or until beef is tender.
About 15 minutes before steak is ready to serve, microwave vegetables as directed on bag. Stir into beef mixture.