Place the two remaining tablespoons of butter in the same skillet. Add the onion and the sugar. Season with salt and pepper. Let it stir-fry until the onion is translucent and fragrant, about 2 – 3 minutes. Remove from the heat and add the sour cream, chipotle chili pepper and tomatillo sauce; stir until combined. Add the green beans and corn; stir until everything is covered in sauce. Add the mozzarella cheese.