Rinse the cress and blanch them in boiling water for a minute, drain them and squeeze them well. Mix the cress and some butter in a food processor then put it in a bowl. Clean the leek, removing the roots and the tougher leaves; peel the shallots, rinse it and chop, then chop both the leek and the shallot.Place the leek and the shallot in a pan of water, bring to the boil and simmer until the liquid is reduced to a table spoon. Add the cream and reduce to half. Remove the pan from the heat and add the mix a bit at a time folding in the cutter and cress with a whisk.Add salt and pepper and keep it warm au-bain-marie. In the meanwhile cook the past and drain it al dente, arrange it in a serving dish and pour the cress sauce over it, mix well and serve immediately.