Bring a large pot of salted water to a boil and cook the tagliatelle until al dente. Reserve a cup of pasta water before draining.
Meanwhile, heat olive oil in a large skillet over medium heat. Add the chopped onion and sauté until soft and translucent.
Add the garlic and sliced mushrooms. Cook for 8-10 minutes until the mushrooms are golden and their moisture has evaporated.
Lower the heat and stir in the double cream. Let it simmer gently for 3-4 minutes, then season with salt and pepper.
Toss the cooked pasta into the sauce. Add a splash of the reserved pasta water if needed to loosen the sauce.
Serve warm, topped with parsley and parmesan if using.