Prepare your mushrooms by gently brushing them with a mushroom brush or paper towel to remove any excess dirt. Once cleaned, slice them thinly.
In a large pot, cook the fettuccine as per package directions in heavily salted water. While the pasta is cooking, reserve 1 cup of the cooking water and set it aside.
In a separate pot or extra-large pan on medium-high heat, warm 1 tablespoon of butter and olive oil, add sliced mushrooms and cook until lightly browned, released their juices, and reduced in size, about 5-6 minutes. Once cooked, season with a pinch of salt and pepper, toss, then remove the mushrooms from the pot, transfer them to a plate and set aside.
In the same pot, reduce heat to medium, melt the remaining tablespoon of butter, add minced shallot and garlic and cook for 2-3 minutes until tender and translucent.
Once the shallot and garlic are tender, add 1/3 cup of the pasta cooking water, use a wooden spoon or spatula to scrape up any tasty bits from the bottom of the pot, and bring to a bubble.
Reduce the heat to low, add heavy cream and parmesan cheese, and whisk until the cheese is melted and the sauce is smooth. Taste and season with additional salt and black pepper as needed.
Turn off the heat, add the cooked fettuccine, mushrooms, and parsley to the pot with sauce and then, using tongs, toss everything together, adding additional splashes of the pasta cooking water to loosen the sauce as needed until a rich and velvety texture is reached.
Serve the pasta immediate with an extra sprinkle of parmesan cheese and parsley, any leftover pasta can be stored in an airtight container in the refrigerator for up to 5 days or in the freezer for up to 3 months.