01 - Preheat oven to 400°F. Lightly grease a medium baking dish with butter or cooking spray.
02 - Cook macaroni according to package directions until al dente. Drain thoroughly and set aside.
03 - In a saucepan over medium heat, melt 2 tbsp butter. Whisk in flour and cook for 1 minute, stirring constantly to form a smooth paste.
04 - Gradually pour in milk while whisking continuously. Continue whisking for 4-5 minutes until sauce thickens and coats the back of a spoon.
05 - Reduce heat to low. Stir in cheddar, mozzarella, Dijon mustard, black pepper, and salt until cheese is completely melted and sauce is velvety smooth.
06 - Fold in cooked macaroni, chopped kimchi, kimchi juice, spring onions, and gochujang. Mix thoroughly until pasta is evenly coated.
07 - Pour the mixture into the prepared baking dish, spreading evenly.
08 - In a small bowl, combine panko breadcrumbs, melted butter, and sesame seeds. Mix until evenly distributed.
09 - Sprinkle crumble mixture evenly over the macaroni. Bake for 15 minutes until golden brown and bubbly.
10 - Let rest for 5 minutes before serving to allow the sauce to set slightly.