Carefully wash then clean the porcini mushrooms. Take half and cut into cubes.
Cut the other half into wide and thin slices. Now finely chop the shallot and soften in a non-stick frying pan with 4 tablespoons of olive oil and a nob of butter.
Add the pieces of porcini, but not the slices.
Sprinkle them in the frying pan, be careful not to overcook: mushrooms, like fish, do not need a lot of cooking (7/8 minutes is enough).
Season with salt and pepper and add the porcini slices, tossing for a couple of minutes over a high heat, then turn the heat off.
Cook the tagliatelle in plenty of salt water and add to the pan together with the mushrooms.
Toss energetically for a few minutes, turn off the heat and serve.