Put the dried porcini mushrooms in a bowl and cover with boiling water.

Finely chop the carrots, celery, leek, onion, rosemary and garlic and collect them in a thick-bottomed pot. Add the extra virgin olive oil and a generous pinch of salt.

Stir and cook on low heat for about 10 minutes, until all the vegetables are soft.

Add the diced butternut squash. Cook for about 10 minutes.

Remove the dried mushrooms from the water and squeeze out the excess water. Keep the water aside and add the porcini into the pot.

Add also the lentils, stir to mix and cover them with half of the mushroom water, where you'll have previously melt a spoonful of tomato paste.

Cook over low heat for about 25-30 minutes, checking often that the lentils do not get too dry and adding the remaining water. Season with salt and pepper when the lentils are almost ready.

The ragu will be ready when it will be thick and the lentils soft and almost melting into the sauce.

Cook the tagliatelle in a large pot of salted boiling water, drain al dente and toss with the lentil ragu, finishing with some grated Parmigiano Reggiano.