Cover the lentils with plenty of water and bring to the boil on a low fire, depending on the quality, not too soft for 20 - 30 minutes. Drain well. In the meantime, cut the Tilsiter into thin sticks.
Mix the balsamic vinegar, mustard, salt and pepper, then add the olive oil and whisk everything into a creamy sauce. Mix in the onions and parsley.
Mix the lentils, cheese and radishes with the dressing and let steep for about 15 minutes before serving.