01 - Preheat oven to 390°F (200°C). Lightly grease a medium baking dish.
02 - Boil elbow macaroni in salted water until just al dente. Drain and reserve.
03 - In a medium saucepan over moderate heat, melt butter. Whisk in flour and cook, stirring, for 1 minute to create a smooth roux.
04 - Gradually add milk, whisking continuously until smooth and thick—approximately 4 to 5 minutes.
05 - Reduce heat and stir in sharp cheddar, mozzarella, and cream cheese. Mix until fully melted and silky.
06 - Blend in gochujang, Dijon mustard, smoked paprika, and black pepper. Adjust seasoning with salt.
07 - Fold chopped kimchi, kimchi juice, and sliced spring onions into cheese sauce.
08 - Add drained macaroni to sauce. Mix thoroughly so all pasta is well coated. Transfer mixture to prepared baking dish.
09 - In a bowl, combine panko breadcrumbs, toasted sesame seeds, melted butter, and sesame oil. Stir until evenly combined.
10 - Sprinkle breadcrumb mixture evenly across the top of macaroni.
11 - Bake in preheated oven for 15 to 20 minutes, or until topping is golden and cheese is bubbling. Let cool slightly before serving.