Preheat the oven to 350°. Remove the stems from mushrooms and dice.
Melt the butter in a large skillet. Add the mushroom stems, red bell pepper, and onion. Saute until softened. Season with umami seasoning. Remove from heat.
Stir in the sour cream and bread crumbs.
Fill the mushroom caps and bake for 30 minutes.
Remove from oven and sprinkle with cheddar cheese.
Return to oven and bake for 10 minutes more, or until cheese is melted and starting to brown.