Korean Corn Cheese

Name: Korean Corn Cheese
Cuisine: Asian, Korean
Ingredients: 2 tablespoons (30ml) water; 2 teaspoons (5g) Chinese mustard powder; 6 tablespoons (3 ounces; 85g) unsalted butter; 2 tablespoons (30g) doenjang; 1 large yellow onion (about 10 ounces; 285g), finely chopped; 6 ears fresh corn, shucked, kernels removed (about 6 cups; 800g corn kernels) (see note); 1/2 teaspoon Diamond Crystal kosher salt; for table salt use half as much by volume; 6 ounces (170g) low-moisture mozzarella, shredded; 2 scallions (30g), thinly sliced; Coarse sea salt, such as Maldon or fleur de sel (optional); 12-inch cast iron skillet