Cook the fettuccine according to package instructions in a large pot of salted boiling water, until al dente.

In a large skillet, heat the olive oil over medium heat. Add the chopped onion and sauté until translucent, about 3-4 minutes.

Add the minced garlic and cook for an additional minute, stirring constantly until fragrant.

Add the sliced mushrooms to the skillet and cook until they are browned and their moisture has evaporated, approximately 7-10 minutes.

Pour in the heavy cream, stirring to combine, and let it simmer for 3-4 minutes until slightly thickened.

Stir in the grated Parmesan cheese until melted and the sauce is creamy. Season with salt and black pepper to taste.

Add the drained fettuccine to the skillet, tossing gently to coat the pasta evenly with the sauce. If necessary, add reserved pasta water a little at a time to reach the desired sauce consistency.

Serve immediately, garnishing with fresh parsley for a pop of color and added freshness.