01 -  Set oven to 400°F (200°C) and allow to preheat while preparing other components. 
 02 -  Boil macaroni in a large pot of salted water until just al dente. Drain thoroughly and set aside. 
 03 -  Melt unsalted butter in a large oven-safe skillet over medium heat. Whisk in flour and cook for 1 minute until lightly bubbling. 
 04 -  Gradually whisk whole milk into the roux, stirring constantly until smooth. Simmer for 3 to 4 minutes until slightly thickened. 
 05 -  Reduce heat to low. Stir in cheddar cheese, Gruyère cheese, Dijon mustard, garlic powder, salt, and black pepper. Mix until cheeses are melted and sauce is uniform. 
 06 -  Add drained macaroni to the skillet and stir gently, ensuring all pasta is well coated in cheese sauce. 
 07 -  In a medium bowl, combine sourdough bread chunks, melted unsalted butter, parsley (if desired), and grated Parmesan. Toss to coat evenly. 
 08 -  Distribute the sourdough mixture evenly over the macaroni in the skillet. 
 09 -  Transfer skillet to oven and bake for 15 to 18 minutes, until topping is golden brown and crisp. 
 10 -  Allow the skillet to cool for 5 minutes before portioning and serving.