Simmer the lentils in 450 ml of boiling salted water, covered, for 5-6 minutes. Drain and let cool off. In the meantime wash parsley, dab dry and cut the leaves into fine strips.
Peel onions and garlic. Chop the onion finely, press the garlic through a garlic press. Squeeze lemon and mix the juice with salt, pepper, onions and garlic. Beat the oil into it.
Mix the lentils and parsley. Add the marinade to the salad just before serving.