Dip the porcini mushrooms in the water and keep them aside.
Brown the sliced onions in olive oil, add the mushrooms than the porcini mushrooms and stir 3-4 mins; you can add the white wine until it evaporates, then add the chicken broth and the porcini water, stir constantly.
Add the crème fraiche and the parmesan cheese, season with salt and pepper. Add a spoon of corn flour diluted in ¼ cold water.
Cook the tagliatelle al dente and mix with the sauce.