Place bread in a food processor; pulse 10 times or until coarse crumbs measure 1 cup. Set aside.
Remove stems from mushrooms; chop enough stems to measure 1/4 cup; set aside. Discard remaining stems.
Remove brown gills from the undersides of mushrooms, using a spoon; discard gills. Coat both sides of mushroom caps with cooking spray. Place mushroom caps, gill sides up, on a baking sheet; sprinkle evenly with salt and pepper.
Heat oil in a large nonstick skillet over medium-high heat. Add bell pepper, reserved chopped mushroom stems, and garlic; sauté 4 minutes. Stir in breadcrumbs and fines herbs; cook 2 minutes or until breadcrumbs are toasted. Spoon mushroom mixture evenly into mushroom caps; sprinkle evenly with cheese. Bake at 375° for 15 minutes or until mushroom caps are tender.