In a large skillet cook beef over medium-high heat until no longer pink, 5-6 minutes, breaking into crumbles; drain and transfer to a 5-qt. slow cooker. Add onions and mushrooms to skillet; cook over medium-high heat until vegetables are tender, 8-10 minutes. Add garlic; cook 1 minute longer. Stir in condensed soup, salt and pepper. Place vegetable mixture in slow cooker; add green beans and stir to combine. Top with Tater Tots and cheese. , Cook, covered, on low 6 hours. Let stand, uncovered, 15 minutes before serving.