1. Preheat oven to 400ºF. Mist a rimmed baking sheet with cooking spray. Set a wire rack over a second rimmed baking sheet. Wipe mushrooms with a damp kitchen towel. Remove stems from mushrooms; reserve. Brush mushroom caps with 2 Tbsp. oil and sprinkle with salt and pepper. Place stem-side down on baking sheet and bake for 10 minutes, until mushrooms are softened and exude juices. Remove from oven and transfer mushrooms caps, stem-side down, to the wire rack. Let cool. Discard any accumulated liquid.
2. Trim ends and finely chop enough of the reserved stems to make 1 cup. Warm remaining 2 Tbsp. oil in a large skillet over medium heat. Add sausage and cook, breaking it up into small pieces, until no longer pink, about 4 minutes. Add chopped mushroom stems, onion, red bell pepper and garlic and sauté for 5 minutes, until vegetables have softened. Remove from heat and stir in cream cheese until melted. Add bread crumbs, parsley, Italian seasoning and 1/2 cup Parmesan. Season with salt and pepper. Add up to 1/4 cup chicken broth if mixture appears dry.
3. Mist a 9-by-13-inch baking dish with cooking spray. Set mushroom caps in dish, stem side up. Mound filling mixture on mushrooms and sprinkle with remaining Parmesan. Bake until topping is golden, 25 to 30 minutes. Let cool for 10 minutes before serving.