Cook the Pasta: Bring a large pot of salted water to a boil and cook the pasta according to the package instructions until al dente. Drain and set aside, reserving 1/2 cup of pasta water.
Sauté the Vegetables: While the pasta cooks, heat olive oil and butter in a large skillet over medium heat. Add the onion and garlic and sauté for about 2 minutes until softened and fragrant.
Cook the Mushrooms: Add the sliced mushrooms to the pan and cook for 5-7 minutes until they release their moisture and become golden brown.
Make the Sauce: Reduce the heat to low and pour in the heavy cream. Stir to combine and let the sauce simmer for a few minutes. Add the Parmesan cheese and stir until melted and smooth.
Combine and Serve: Toss the cooked pasta into the sauce, adding reserved pasta water if necessary to thin out the sauce. Season with salt and pepper to taste. Garnish with fresh parsley and extra Parmesan if desired.