Bring a large pot of salted water to a boil. Add the pasta and cook as the label directs. Reserve 1 cup cooking water, then drain.
Meanwhile, in a large skillet, heat the butter, thyme and garlic over medium heat until the garlic is tender, about 2 minutes. Add the mushrooms and cook until they release their water and begin to brown, about 7 minutes. Season generously with salt and pepper.
Add the flour to the skillet and cook, stirring, 2 to 3 minutes. Whisk in the milk, heavy cream and mustard. Season with salt and pepper. Bring to a low simmer and continue whisking until the mixture thickens enough to coat the back of a spoon, about 3 minutes. Stir in the fontina and Parmesan until melted.
Add the pasta to the sauce and toss to coat. If the sauce thickens as it sits, toss in a splash of the reserved cooking water. Divide the mac and cheese among bowls and top each with a slice of taleggio.