Twist mushroom stems to remove from mushroom caps. Finely chop enough stems to measure 1/3 cup. Reserve mushroom caps.
Melt 2 tablespoons butter in 10-inch skillet over medium-high heat. Cook chopped mushroom stems, onions and bell pepper in butter about 3 minutes, stirring frequently, until onions are softened; remove from heat. Stir in bread crumbs, Italian seasoning, salt and pepper. Add melted butter to dampen if necessary. Fill mushroom caps with bread crumb mixture.
Melt 1 tablespoon butter in rectangular pan,(I use non stick baking spray) 13x9x2 inches, in oven. Place mushrooms, filled sides up, in pan. Drizzle with olive oil and if desired Sprinkle with cheese. Bake 15 minutes.
Set oven control to Broil. Broil mushrooms with tops 3 to 4 inches from heat about 2 minutes or until tops are light brown. Serve hot.