Bring a large pot of salted water to a boil over high heat.
Add the butter to a large skillet set over medium-high heat. Once the butter has melted, add the mushrooms and cook, stirring, until the mushrooms have softened slightly, about 3 minutes. Add the garlic and continue cooking an additional 2 minutes.
Stir in the stock, Dijon mustard, Worcestershire sauce and heavy cream. Reduce the heat and simmer the sauce until it has thickened slightly, 8 to 10 minutes, then stir in the thyme leaves.
While the sauce is simmering, add the pasta to the boiling water and cook until al dente according to package directions. Reserve 1/2 cup pasta water, then drain the pasta.
Add the pasta to the skillet with the sauce along with 1/4 cup of the reserved pasta water, tossing to coat. Add more pasta water as needed until the sauce coats the pasta.
Taste and season with salt and pepper, then serve topped with freshly grated Parmesan cheese and additional fresh thyme.