01 - Preheat the oven to 400°F if preparing the baked version with crispy topping.
02 - Bring a large pot of salted water to a boil. Cook macaroni according to package directions until al dente. Drain thoroughly 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 continuously, until the mixture thickens and coats the back of a spoon, approximately 3-4 minutes.
05 - Reduce heat to medium-low. Add Boursin cheese, cheddar, salt, pepper, and nutmeg. Stir until all cheeses are completely melted and the sauce is smooth and velvety.
06 - Add cooked macaroni to the cheese sauce. Stir gently until pasta is evenly coated with the creamy mixture.
07 - In a small bowl, combine panko breadcrumbs, melted butter, and grated Parmesan. Mix until evenly distributed.
08 - Pour mac and cheese into a greased baking dish. Sprinkle the crumble topping evenly over the surface. Bake for 10-12 minutes until golden brown and bubbly.
09 - Serve hot, garnished with chopped fresh parsley if desired.