01 - Preheat oven to 350°F (177°C). Lightly grease a 2-liter baking dish with butter.
02 - Boil macaroni until al dente according to package instructions; drain and set aside.
03 - In a large skillet over medium heat, sauté chopped mushrooms in truffle oil until tender and fragrant, about 5 minutes.
04 - Melt butter in a saucepan over medium heat. Add flour and whisk continuously for 2 minutes until pale golden.
05 - Gradually add whole milk to roux, whisking constantly to prevent lumps. Stir in salt, white pepper, and nutmeg. When mixture thickens, reduce heat and add cheddar, Gruyère, and Parmesan cheese, stirring until smooth.
06 - Fold cooked macaroni and sautéed mushrooms into cheese sauce. Mix until noodles are evenly coated.
07 - Transfer the mixture into prepared baking dish. Toss panko breadcrumbs with melted butter and parsley, then sprinkle evenly over macaroni.
08 - Place in oven and bake for 25 minutes, or until golden brown and bubbling.
09 - Allow to cool for 10 minutes before serving for improved texture and flavor.