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.
If you make this dip ahead of time, leave the baking until right before serving. Cover it well and store it in the fridge. Bake for 45 minutes out of the fridge, or until nice and bubbling hot.
If you manage to have some leftovers, transfer them to a sealable container and store in the fridge for 3-4 days. You can reheat it in the microwave, but I would suggest putting it back in the oven so that you don’t lose that nice, brown, crisp exterior.