Melt butter in a large pot. Whisk in flour — cook 1 minute (this is a roux, the same French technique I use in about 40% of my recipes). Gradually pour in milk, whisking constantly to prevent lumps. Cook 5-6 minutes until thickened. Remove from heat. Stir in cheddar, gruyère, and gochujang until melted and smooth. The sauce should be orange-red, creamy, and glossy. Taste: it should be cheesy, savoury, and have a warm, building spiciness from the gochujang that's completely different from hot sauce heat.
Combine and Bake
Cook pasta 1 minute short of al dente. Drain. Stir into the cheese sauce. Pour into a baking dish. Top with the panko mixture (panko tossed with melted butter and sesame seeds — the sesame seeds are a nod to the Korean element). Bake at 190C / 375F for 20-25 minutes until bubbly and the panko topping is golden. The sesame-panko crust adds crunch and ties the Korean-American fusion together visually and texturally.
Notes
Fridge 4-5 days. Reheat in the oven covered at 175C / 350F for 15 minutes, or microwave with a splash of milk. The panko topping softens overnight — if re-crisping matters, add fresh panko before reheating in the oven. Freezes for 2 months.