Mix first 5 ingredients and, if desired, pepper flakes. In a large skillet, cook beef with onion, garlic and ginger over medium heat until no longer pink, 5-7 minutes; crumble the meat. Transfer to a 3- or 4-qt. slow cooker. Add mushrooms, carrot, celery and pepper; stir in sauce mixture., Cook, covered, on low until vegetables are tender and flavors are blended, 3-4 hours. Serve in lettuce leaves; sprinkle with green onions.