01 - Preheat oven to 400°F and lightly grease a medium baking dish with butter or cooking spray.
02 - Cook elbow macaroni according to package directions until al dente. Drain thoroughly and set aside.
03 - In a saucepan over medium heat, melt 2 tablespoons butter. Whisk in flour and cook for 1 minute, stirring constantly to form a roux.
04 - Gradually pour in milk while whisking continuously. Continue whisking until sauce thickens and becomes smooth, about 4-5 minutes.
05 - Reduce heat to low. Stir in sharp cheddar, mozzarella, Dijon mustard, black pepper, and salt until cheese is completely melted and sauce is smooth and creamy.
06 - Add cooked macaroni, chopped kimchi, kimchi juice, sliced spring onions, and gochujang to the cheese sauce. Fold gently until well combined.
07 - Pour the mac and cheese mixture into the prepared baking dish, spreading evenly.
08 - In a small bowl, combine panko breadcrumbs, melted butter, and sesame seeds. Mix until crumbs are evenly coated.
09 - Sprinkle breadcrumb mixture evenly over the macaroni. Bake for 15 minutes until golden brown and bubbly.
10 - Remove from oven and let rest for 5 minutes before serving to allow sauce to set slightly.