Place butter in a 5-qt. slow cooker. Top with sweet and red onions; sprinkle with pepper. Cook, covered, on low until onions are tender, 5-6 hours., Stir in broth, water, wine, herbs and Worcestershire sauce. Cook, covered, on low until flavors are blended, 2-3 hours. Remove herb sprigs and bay leaf., To serve, preheat broiler. Place bread on a baking sheet; broil 4 in. from heat until lightly toasted, 1-2 minutes per side. Top bread with cheese; broil until cheese is melted, 1-2 minutes. Divide soup among eight bowls; top with cheese toasts and serve immediately.