Cook the fettuccine according to package instructions in a large pot of salted boiling water until al dente. Drain and set aside, reserving 1/2 cup of the pasta water.
In a large skillet, heat the olive oil over medium heat. Add the diced onion and sauté for about 3-4 minutes until softened.
Stir in the minced garlic and sliced mushrooms. Cook for another 5-7 minutes until the mushrooms are browned and have released their moisture.
Add the dried thyme, salt, and black pepper to the skillet, stirring to combine.
Pour in the heavy cream and bring to a gentle simmer. Cook for 2-3 minutes until the sauce thickens slightly.
Stir in the grated Parmesan cheese until melted and well combined. If the sauce is too thick, add a little reserved pasta water until desired consistency is reached.
Add the cooked fettuccine to the skillet, tossing to coat the pasta evenly with the sauce.
Serve immediately, garnished with fresh parsley.