Spray inside of 4-quart slow cooker and large skillet with cooking spray. Heat large skillet over medium-high heat. Sprinkle beef with 1/2 teaspoon salt and pepper; cook 3 to 5 minutes or until browned on each side, turning once. Place beef in slow cooker.
Add onion and garlic to skillet; cook until fragrant. Add beer, stirring to loosen brown bits from bottom of skillet. Add tomato paste and brown sugar to skillet; stir until blended. Pour mixture over beef in slow cooker.
Cover; cook on LOW 8 hours or HIGH 4 hours. Transfer beef to cutting board and cut into slices. Stir remaining 1/4 teaspoon salt into sauce. Serve beef on buns with sauce spooned over beef or for dipping.