Cook the Pasta:

1. Boil the Pasta: Bring a large pot of salted water to a boil. Add the pasta and cook according to the package directions until al dente. Drain, reserving about ½ cup of the pasta cooking water. Set aside.

Prepare the Mushroom Sauce:

2. Sauté the Onion and Garlic: In a large skillet, heat the olive oil over medium heat. Add the chopped onion and cook until softened and translucent, about 3-4 minutes. Add the minced garlic and cook for another 30 seconds until fragrant.
3. Cook the Mushrooms: Add the sliced mushrooms to the skillet and cook, stirring occasionally, for 5-7 minutes, until the mushrooms release their moisture and become golden brown. Season with salt and pepper.
4. Add the Cream and Broth: Pour in the heavy cream and vegetable broth. Stir to combine, allowing the sauce to simmer for 3-4 minutes until it thickens slightly. Add the thyme and soy sauce (if using), and continue to cook for another 1-2 minutes to let the flavors meld together.

Combine the Pasta and Sauce:

5. Toss Pasta with Sauce: Add the cooked pasta to the skillet with the mushroom sauce. Toss to combine, adding a bit of the reserved pasta cooking water if the sauce needs to loosen up.
6. Finish the Dish: Stir in the grated Parmesan cheese, if using, and toss until the pasta is well-coated with the sauce. Adjust seasoning with salt and pepper to taste.

Garnish and Serve:

7. Serve: Divide the pasta among serving plates and sprinkle with freshly chopped parsley for color and freshness. Serve immediately with extra Parmesan cheese on the side.