Preheat oven to 350 degrees F.
Melt the butter in a large (10 to 12-inch) skillet over medium heat, then add the chopped mushroom stems, onion, pepper and garlic.
Saute until the mixture is softened and most of the liquid has evaporated, about 10 minutes. Remove from the heat and stir in the remaining ingredients.
Fill each mushroom cap with the stuffing and bake until the mushrooms are dark and the stuffing is golden, about 25 minutes. Serve warm.
Note: The stuffing can be made up to 2 days ahead of time. The mushrooms can be assembled, placed in a baking dish, covered and refrigerated for up to 1 day before baking.