01 - Preheat oven to 400°F. Lightly grease a medium baking dish.
02 - Cook macaroni according to package instructions until just al dente. Drain and set aside.
03 - In a saucepan over medium heat, melt 2 tbsp butter. Whisk in flour and cook for 1 minute.
04 - Gradually pour in milk, whisking constantly until smooth and thickened, about 4–5 minutes.
05 - Reduce heat to low. Stir in cheddar, mozzarella, mustard, black pepper, and salt until cheese is melted and sauce is smooth.
06 - Fold in the cooked macaroni, chopped kimchi, kimchi juice, spring onions, and gochujang if using. Mix thoroughly.
07 - Pour the mixture into the prepared baking dish.
08 - In a small bowl, combine panko breadcrumbs with 1 tbsp melted butter and sesame seeds. Sprinkle evenly over the macaroni.
09 - Bake for 15 minutes, or until golden and bubbling.
10 - Let rest for 5 minutes before serving to allow the sauce to set.