01 - Set oven to 400°F (200°C).
02 - Boil macaroni in salted water until just al dente; drain and set aside.
03 - In a large oven-safe skillet over medium heat, melt 2 tablespoons of butter. Add flour and whisk for 1 minute.
04 - Gradually add milk, whisking constantly to create a smooth sauce. Simmer for 3–4 minutes until slightly thickened.
05 - Lower heat. Add cheddar, Gruyère, Dijon mustard, garlic powder, salt, and pepper; stir until cheese is fully melted and sauce is smooth.
06 - Fold drained macaroni into the skillet, ensuring noodles are evenly coated.
07 - In a mixing bowl, toss sourdough bread chunks with melted butter, parsley (if using), and Parmesan cheese.
08 - Distribute sourdough mixture evenly over macaroni in skillet.
09 - Place skillet in preheated oven and bake for 15–18 minutes, until topping is crisp and golden.
10 - Allow to cool for 5 minutes before serving.