Spray 5- to 6-quart slow cooker with cooking spray. Sprinkle beef with salt and pepper. Heat 12-inch nonstick skillet over high heat; add 2 teaspoons oil. Cook beef about 3 minutes on each side. Place beef in slow cooker; add garlic. In blender, place salsa ingredients. Cover; blend until smooth. Pour half of salsa into slow cooker. Cover; cook on High heat setting 4 hours to 4 hours 30 minutes. In same skillet, heat 1 teaspoon oil over high heat. Stir in onions. Reduce heat to medium-low; cook about 15 minutes, stirring frequently, until onions are caramelized. Remove from skillet to bowl. Cover; refrigerate.