Preheat oven to 375F
Butter a 3 quart casserole dish
Warm milk over medium heat. In a separate large stock pot, melt butter until bubbling, then add flour and cook, stirring for 1 minute. Add warm milk to stock pot slowly.
Whisk mixture constantly until it thickens and starts to bubble
Remove pot from heat, add 300 g (~3 cups) cheddar and 170 g (~1.5 cups) gruyere. Add nutmeg, black pepper, red pepper, and salt; whisk until smooth
Cook macaroni a bit less than al-dente. Mix together with cheese sauce and pour mixture into casserole
Sprinkle remaining cheese and optional topping over casserole. Bake for ~30 minutes