Serrano Sauce: Rinse tomatoes and serrano pepper. Peel onion and garlic. Place all on a cookie sheet and roast in a 350 degree oven for 15 minutes. Remove and blend all together in a food chopper or blender. Cook over medium heat in a saucepan for a few minutes. Set aside.
Rinse mushrooms and chop about 1/4 of the end of stems. (Removes rough ends) Break off stems with fingers and chop coarsely. Set aside.
In a skillet over medium heat, add butter and allow to melt. Add pepper, onion and garlic. Cook until onion is translucent. Add black beans, corn and all seasoning. Cook for 2 minutes. Remove from heat and allow to cool down for a few minutes in a bowl.
Now add the cilantro, sour cream and mayonnaise to bowl. Mix well.
Grease a baking sheet or baking pan and place mushroom caps. Fill all caps with mixture and top with shredded mozzarella. Bake for 10-15 minutes at 350 degrees F or 176 degrees C.
When mushrooms are done top with tomato serrano sauce. Serve immediately.