Step 1: Cook the macaroni according to package directions in well-salted water until al dente. Drain well and set aside.
Step 2: In a large, heavy-bottomed pot or Dutch oven, melt the unsalted butter over medium heat. Once melted, whisk in the all-purpose flour. Cook, whisking constantly, for 1-2 minutes until a pale golden roux forms.
Step 3: Gradually whisk in the warmed whole milk, a little at a time, until smooth. Continue to whisk constantly, ensuring there are no lumps. Bring the mixture to a gentle simmer, whisking, and cook for 3-5 minutes, or until the sauce has thickened to coat the back of a spoon.
Step 4: Reduce the heat to low. Add the softened Boursin Garlic & Fine Herbs cheese, stirring until it’s completely melted and incorporated. Then, add the grated sharp cheddar cheese and Gruyère cheese, handful by handful, stirring continuously until each addition is fully melted before adding more.
Step 5: Stir in the Dijon mustard, salt, freshly ground black pepper, and a pinch of cayenne pepper (if using). Taste and adjust seasonings as needed.
Step 6: Add the cooked and drained macaroni to the cheese sauce, stirring gently until every piece is thoroughly coated.
Step 7: Serve immediately as a standalone meal or a spectacular side dish. Garnish with fresh parsley or chives, if desired.