Step 1: Cook the pasta. Bring a large pot of salted water to a rolling boil. Add the elbow macaroni and cook according to package directions until al dente. Drain well and set aside.
Step 2: Prepare the béchamel. In a large, heavy-bottomed pot or Dutch oven, melt the unsalted butter over medium heat. Once melted, whisk in the flour and cook for 1-2 minutes, stirring constantly, to create a roux. Do not let it brown too much.
Step 3: Add the milk. Gradually whisk in the warm milk, a little at a time, ensuring no lumps form. Continue to whisk as the sauce comes to a gentle simmer and thickens. This usually takes about 5-7 minutes. Once thickened, remove from heat.
Step 4: Incorporate the cheese. Stir in the nutmeg, Dijon mustard, salt, and pepper. Gradually add the grated Gruyère, Fontina, and white cheddar cheeses, stirring until each addition is completely melted and smooth before adding more. The sauce should be luxuriously creamy.
Step 5: Add truffle oil. Stir in 1 tablespoon of white truffle oil. Taste and add an additional tablespoon if you desire a stronger truffle flavor.
Step 6: Combine pasta and sauce. Add the cooked and drained macaroni to the cheese sauce. Stir gently to ensure all the pasta is thoroughly coated with the rich sauce.
Step 7: Bake (optional). For an extra layer of flavor and a crispy topping, transfer the truffle mac and cheese mixture to a 9x13 inch baking dish. Sprinkle with additional grated cheese or panko breadcrumbs, if desired. Bake in a preheated oven at 375°F (190°C) for 15-20 minutes, or until bubbly and golden brown on top.
Step 8: Serve and enjoy. Let the baked mac and cheese rest for a few minutes before serving. Garnish with fresh chives or parsley, if desired. Enjoy your decadent Truffle Mac and Cheese!