- Preheat oven to 375°F
- Butter a 3-quart casserole dish; set aside
- Warm milk in a saucepan over medium heat.
- Melt 6 tablespoons butter in a high-sided skillet over medium heat.
- When butter bubbles, add flour. Cook, whisking 1 minute
- While whisking, add hot milk a little at a time to keep mixture smooth. Continue cooking, whisking constantly, until mixture bubbles and thickens, 8-12 minutes
- Remove pan from heat. Stir in slat, nutmeg, peppers, 3 cups cheddar cheese, and 1 1/2 cups Gruyere; set the cheese sauce aside
- Cook the macaroni until slightly underdone (about 1-2 minutes less than the directions say
- Transfer the macaroni to a colander; rinse under cold running water, and drain well
- Stir the macaroni into the cheese sauce
- Pour mixture into prepared dish
- Sprinkle remaining cheeses and panko crumbs
- Dot top of dish with remaining 2 tablespoons of butter
- Bake until golden, about 30-40 minutes