Bring a pot of salted water to a boil and cook the tagliatelle until al dente. Reserve a little pasta water before draining.
While the pasta cooks, heat olive oil in a large pan over medium heat.
Add the chopped onion and sauté for 3 minutes until softened.
Stir in the garlic and mushrooms. Cook until the mushrooms release their moisture and start to brown.
Pour in the white wine and let it simmer for about 2 minutes.
Lower the heat and stir in the cream. Simmer for another 2-3 minutes until the sauce thickens slightly.
Season with salt and black pepper. If the sauce gets too thick, stir in a splash of reserved pasta water.
Toss the cooked tagliatelle into the sauce, mixing well to coat every strand.
Serve immediately with chopped parsley and grated parmesan, if using.