Spray 5- to 6-quart slow cooker with cooking spray. In 12-inch nonstick skillet, cook bacon over medium-high heat, stirring occasionally, until crisp. Place bacon in cooker. Discard all but 1 tablespoon bacon fat in skillet. Cook beef in bacon fat 2 to 3 minutes, stirring occasionally, until brown. Stir onion into beef. Cook 1 minute, stirring occasionally. Spoon mixture into cooker.