1. For filling, place bacon in a large nonstick skillet. Cook over medium heat for 8 to 12 minutes or until crisp, turning often. Drain bacon on paper towels; reserve 1 Tbsp. drippings in skillet. Add butter to skillet; heat until melted. Add onions; sprinkle with thyme, brown sugar, and black pepper. Cook over medium-low heat for 30 to 35 minutes or until onions become very soft and begin to caramelize, stirring occasionally.
2. Increase heat to medium-high. Stir in beef broth. Simmer for 1 to 2 minutes or until broth has evaporated. Remove from heat. Finely crumble bacon and stir into onion mixture. If desired, cool filling and transfer to a tightly covered container; refrigerate up to 24 hours.
3. To serve, preheat oven to 350°F. Bring onion mixture to room temperature. Clean mushrooms; remove and discard stems. Place mushrooms, stem sides up, on a wire rack in a large rimmed baking pan. (For fewer servings, place mushrooms in a baking pan lined with parchment paper.)
4. Spoon onion mixture into mushroom caps. Top with cheese. Bake for 12 to 15 minutes or until heated through. Turn on broiler and broil 6 inches from heat for 10 to 20 seconds or until cheese is golden. Transfer to a serving platter; sprinkle with parsley.

Each piece contains: 39 calories, 2 g fat, 1 g saturated fat, 0 g trans fat, 6 mg cholesterol, 49 mg sodium, 4 g carbohydrates, 1 g fiber, 2 g sugar, 2 g protein. Daily values: 1% vitamin A, 4% vitamin C, 3% calcium, 1% iron.