1Spray 12-inch skillet with cooking spray; heat over medium heat. Cook beef and onion in skillet 8 to 10 minutes, stirring occasionally, until beef is brown; drain well.
2Stir in refried beans and tomatoes. Cook 3 to 4 minutes, stirring occasionally, until hot. Stir in 3/4 cup of the cheese.
3Spoon dip into serving dish or 1 1/2- to 2-quart slow cooker on Low heat setting. Dip will hold up to 2 hours in slow cooker. Top with remaining 1/4 cup of cheese. Serve with tortilla chips.