Bring a large pot of salted water to a boil.

Add the rigatoni and cook according to package instructions until al dente.

Reserve 1 cup of pasta water, then drain the pasta.

In a large skillet, heat olive oil over medium heat.

Add the chopped onion and sauté until translucent, about 5 minutes.

Stir in the minced garlic and cook for an additional minute.

Add the sliced mushrooms and cook until they release their moisture and become golden brown, about 7-10 minutes.

Pour in the heavy cream and bring to a simmer.

Stir in the grated parmesan cheese and thyme, and season with salt and pepper to taste.

Add the cooked rigatoni to the mushroom sauce, tossing to combine.

If the sauce is too thick, add reserved pasta water a little at a time until desired consistency is reached.

Divide the rigatoni among plates.

Garnish with chopped parsley and additional parmesan if desired.