1. In a small saucepan, cover lentils with water by 1 inch. Bring to a boil. Reduce heat and simmer, partially covered, until lentils are tender but still holding their shape, 20 to 25 minutes. Drain well.

2. Meanwhile, place eggs in a small saucepan; cover with cold water by 1 inch. Bring to a boil; remove from heat and cover. Let stand 12 minutes. Drain, and rinse under cold water. Peel eggs; chop roughly.

3. In a large bowl, whisk together vinegar and oil. Add lentils, cucumber, parsley, and onion; season with salt and pepper. Toss well to coat with dressing. Let cool to room temperature, or serve chilled, topped with chopped eggs.