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 rolling boil. Cook macaroni according to package directions until al dente. Drain thoroughly in a colander and set aside.
03 - In a large saucepan, melt butter over medium heat. Whisk in flour and cook for 1 minute, stirring constantly, until mixture bubbles but does not brown.
04 - Gradually whisk milk into the roux. Cook over medium heat, stirring constantly, for 3-4 minutes until sauce thickens and coats the back of a spoon.
05 - Reduce heat to medium-low. Add Boursin cheese, cheddar, salt, pepper, and nutmeg. Stir continuously until all cheeses are completely melted and sauce is smooth and creamy.
06 - Add cooked macaroni to the cheese sauce. Stir gently but thoroughly until every piece of pasta is evenly coated.
07 - Pour mac and cheese into a greased baking dish. In a small bowl, combine panko, melted butter, and Parmesan. Sprinkle evenly over the surface. Bake for 10-12 minutes until golden brown and bubbly.
08 - Serve immediately while hot, garnished with chopped fresh parsley if desired.