Yield: 6 servings
Adjust oven rack to 6 inches below broiler element and preheat broiler on high. In a small bowl, stir together water and mustard powder to form a smooth, loose sauce that lightly coats the back of a spoon. Set aside.
In a 12-inch cast iron skillet, melt butter over medium heat. Add doenjang and onion and cook, stirring occasionally, until onion is softened and translucent, 5 to 6 minutes.
In a 12-inch cast iron skillet, melt butter over medium heat. Add doenjang and onion and cook, stirring occasionally, until onion is softened and translucent, 5 to 6 minutes.
Sprinkle shredded mozzarella evenly over corn. Transfer skillet to oven, and broil until cheese is fully melted and lightly browned in spots, about 2 minutes. Transfer skillet to a heatproof surface. Drizzle mustard sauce over top, sprinkle with scallions and serve immediately.