1) In a small bowl, prepare the salad dressing: put some oil, lemon juice, salt, finely chopped onion (pepper) and emulsify all the ingredients with a fork.
2) In a colander, wash the lentils under running water, put them in a pot to boil over low heat, with 200 ml of cold water. After 20 minutes, put in some salt. It will take about 10 more minutes to cook, until the water is completely absorbed. Taste them and, if they like them softer, add a little more boiling water and cook them another 5 minutes.
3) Wash and dry the radicchi, cut them and place them in a salad bowl, add the lentils (cold) and season with the emulsion.