01 - Set oven to 400°F (200°C) to ensure even baking of the dish.
02 - Boil macaroni in salted water until just al dente. Drain thoroughly and set aside.
03 - In a large oven-safe skillet over medium heat, melt butter. Whisk in flour and cook for 1 minute until a pale roux forms.
04 - Gradually whisk in milk, stirring constantly for a smooth mixture. Simmer 3–4 minutes until slightly thickened. Lower heat, then stir in cheddar, Gruyère, Dijon mustard, garlic powder, salt, and pepper. Continue mixing until cheese is melted and sauce is homogenous.
05 - Return cooked macaroni to skillet and fold to coat evenly in cheese sauce.
06 - In a mixing bowl, combine sourdough chunks, melted butter, parsley if using, and Parmesan. Toss until bread is evenly coated.
07 - Distribute sourdough mixture evenly across macaroni in skillet.
08 - Transfer skillet to oven and bake for 15–18 minutes until topping is golden and crisp.
09 - Allow to cool for 5 minutes before serving to set the sauce and topping.