Saute the onion, garlic and bacon in the olive oil over a low heat until soft for around 15 minutes, stirring occasionally.
Meanwhile, cook the tagliatelle in a large pan of slightly salted water according to the instructions on the packet.
Drain the pasta in a colander and pour over some boiling water to wash away the starch and prevent the pasta from sticking together.
Stir the mushrooms into the onion and bacon mixture and Saute for five minutes. Stir in the creme fraiche, season to taste and heat through. Remove from the heat and stir in the cooked tagliatelle. Serve immediately.