Preheat oven to 350 F degrees.Melt the butter in a skillet over medium-high heat; add onion and cook until onion is soft and translucent. Add corn and serrano pepper and cook until corn starts to turn golden brown, about 5 minutes. Set aside.In a medium bowl, add cheese, parsley, sour cream and stir. Season with salt and pepper. Add corn mixture and chopped tomatoes and stir until it's well mixed. Pour the mixture into a baking dish and top with additional cheese if preferred.Bake for about 15 minutes or until cheese is bubbling and the top is golden.