Boil a saucepan of water adding a little oil.
Fry the leek and Treviso red lettuce for a couple of minutes in a pan with a spoonful of oil and two spoonfuls of water. Add salt and pepper to taste.
Cook the pasta in boiling water with a level spoonful of salt over a moderate flame for 3 minutes. Before draining, stop the tagliatelle from cooking by adding little cold water. Mix the pasta with the lettuce and leek and serve it immediately.