01 - Set oven temperature to 350°F. Lightly grease an 8x8 inch baking dish.
02 - Boil elbow macaroni in salted water until al dente, then drain and set aside.
03 - Melt butter in a medium saucepan over medium heat, whisk in flour, and cook for 1 minute without browning.
04 - Gradually whisk in milk while stirring constantly; cook until sauce thickens, about 3 to 5 minutes.
05 - Remove from heat and blend in cheddar, Gruyère if using, salt, pepper, and ground mustard until smooth.
06 - Mix the drained macaroni into the cheese sauce until evenly coated.
07 - Transfer the macaroni and sauce mixture into the prepared baking dish.
08 - Mix breadcrumbs, melted butter, and Parmesan cheese, then evenly sprinkle over macaroni.
09 - Bake for 20 minutes until golden and bubbly. Allow to rest for 5 minutes before serving.