Bring some complexity and depth of flavor to the classic dish with mushrooms, Dijon mustard, thyme and two types of cheese: Fontina and Parmesan.
Bring a large pot of water to a boil. Generously salt the water. Add pasta and cook to package instructions. Drain.
In a large skillet, heat butter, garlic, and thyme over medium heat for 2 minutes.
Add mushrooms and cook until mushrooms release their water and begin to brown, about 7 minutes.
Season with 1 teaspoon salt and 1/2 teaspoon freshly ground black pepper. Add flour and cook about 2 minutes.
Whisk in milk, cream, and mustard. Taste and season with salt and pepper if needed. Bring to a low simmer.
Continue whisking until the mixture thickens enough to coat the back of a spoon. Turn off heat. Stir in Parmesan and Fontina until melted. Add pasta and toss to coat. Serve immediately.