Bring a large pot of salted water to a boil. Add 12 ounces of rigatoni and cook according to package instructions until al dente, usually about 10-12 minutes. Reserve ½ cup of pasta water before draining in a colander. Set aside.
In a large skillet, heat 2 tablespoons of olive oil over medium heat. Add the diced onion and cook until translucent and soft, about 5 minutes. Stir in minced garlic and cook for another minute until fragrant.
Add 16 ounces of sliced mixed mushrooms to the skillet. Cook, stirring occasionally, until they release their moisture and become golden brown, about 8-10 minutes. Season with salt and pepper during cooking to enhance their flavor.
Lower the heat and stir in 1 teaspoon of truffle oil, coating the mushrooms evenly. Pour in 1 cup of heavy cream and bring to a gentle simmer. Let it thicken slightly, about 3-5 minutes.
Add the drained rigatoni into the skillet with the creamy mushroom sauce. Toss well to coat all pieces. If the sauce feels too thick, add reserved pasta water a tablespoon at a time until the desired consistency is reached. Stir in 1 cup of grated Parmesan until melted and creamy.
Serve the rigatoni hot, topped generously with freshly grated Parmesan snow. Sprinkle with chopped fresh parsley for a pop of color and freshness. Enjoy immediately for the best texture and flavor.
