Bring a large pot of salted water to a boil. Add the 12 ounces of fettuccine pasta and cook according to package directions until al dente—usually about 10-12 minutes. Once done, reserve about ½ cup of the pasta water, then drain the noodles and set aside.
While the pasta cooks, heat 2 tablespoons of olive oil in a large skillet over medium heat. Add the chopped onion and cook for about 3-4 minutes until softened and translucent. Stir in the minced garlic and cook for another 1-2 minutes, being careful not to let it burn.
Add the sliced mushrooms to the skillet. Cook for 5-7 minutes, stirring occasionally, until the mushrooms release their moisture and start to brown.
Pour in the 1 cup of heavy cream and stir to combine with the mushrooms, onion, and garlic. Bring the mixture to a gentle simmer and let it reduce slightly for 3-5 minutes, stirring occasionally.
Gradually sprinkle in the 1 cup of grated Parmesan cheese, stirring constantly to melt it smoothly into the sauce. Season with salt and black pepper to taste. If the sauce feels too thick, add a splash of the reserved pasta water to loosen it up.
Add the cooked fettuccine to the skillet with the sauce. Toss everything together gently but thoroughly, ensuring the noodles are fully coated in the creamy mushroom Alfredo sauce.
Sprinkle the chopped fresh parsley over the top for a bright, fresh note and a pop of color. Serve immediately while warm and creamy.