01 - Preheat the oven to 400°F if preparing the baked version 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 - Melt butter in a large saucepan over medium heat. Whisk in flour and cook for 1 minute, stirring constantly, until bubbling but not browned.
04 - Gradually whisk in milk, stirring constantly. Cook for 3-4 minutes until the mixture thickens and coats the back of a spoon.
05 - Reduce heat to medium-low. Add Boursin cheese, cheddar, salt, pepper, and nutmeg. Stir until cheeses are completely melted and sauce is smooth.
06 - Add cooked macaroni to the cheese sauce. Stir until pasta is evenly coated.
07 - Pour mac and cheese into a greased baking dish. Mix panko, melted butter, and Parmesan in a small bowl, then sprinkle evenly over the top. Bake for 10-12 minutes until golden and bubbly.
08 - Serve hot, garnished with chopped fresh parsley if desired.