Bring a large pot of salted water to a boil over high heat. Add the tagliatelle and cook according to package instructions until al dente, usually around 8-10 minutes. Drain the pasta, reserving 1 cup of the starchy pasta water, and set it aside.

In a large skillet, heat 2 tablespoons of olive oil over medium heat. Add the diced onion and sliced mushrooms, cooking for about 5 minutes until softened and beginning to brown. Stir in minced garlic and cook for an additional 1 minute until fragrant.

Pour in a cup of white wine and scrape any browned bits stuck to the bottom of the skillet, incorporating them into the sauce. Allow to simmer for about 3-4 minutes, or until reduced by half.

Reduce heat to low and stir in 1 cup of heavy whipping cream along with 4 ounces of cream cheese. Whisk until smooth, allowing the sauce to simmer for about 5 minutes.

Add the cooked tagliatelle to the sauce, gently tossing everything together. Sprinkle half of the grated Parmesan cheese into the pan, mixing well and incorporating the reserved pasta water as needed.

Season with salt and freshly cracked black pepper to taste. Serve hot, garnished with remaining Parmesan cheese and chopped parsley.