01 - Preheat the oven to 400°F if preparing with breadcrumb topping.
02 - Bring a large pot of salted water to a boil. Cook macaroni according to package directions until al dente. Drain and set aside.
03 - In a large saucepan, melt butter over medium heat. Whisk in flour and cook, stirring constantly, for 1 minute until bubbling but not browned.
04 - Gradually whisk in milk. Cook, stirring constantly, until the mixture thickens and coats the back of a spoon, approximately 3-4 minutes.
05 - Lower heat to medium-low. Add Boursin cheese, cheddar, salt, pepper, and nutmeg. Stir until cheeses are completely melted and the sauce is smooth.
06 - Add cooked macaroni to the cheese sauce. Stir until pasta is evenly coated with the sauce.
07 - In a small bowl, combine panko breadcrumbs, melted butter, and Parmesan cheese. Mix until evenly distributed.
08 - Pour mac and cheese into a greased baking dish. Sprinkle breadcrumb mixture evenly over the top. Bake for 10-12 minutes until golden and bubbly.
09 - Serve hot, garnished with chopped fresh parsley if desired.