Preheat oven to 425 degrees. Remove the stems from the mushrooms and chop fine. Heat olive oil in a medium skillet over medium heat. Add the onions and cook until wilted, about 1 minute. Stir in the red pepper and chopped mushroom stems and cook, stirring until tender, about 3 minutes. Remove and cool.
Using 2 tablespoons of the butter, grease a 12-by-18-inch baking pan. Arrange the mushrooms side by side in the pan. Using the remaining butter, dot the top of each mushroom with a small pat of butter. Add the remaining stock, wine, and 2 tablesoons parsley to the pan. Bake until the mushrooms are cooked through and golden brown, about 20-25 minutes.