This salad is hearty enough to be served as a main course. Beluga lentils work well in a cold salad because they're firm and tend to hold their shape. For a wonderful textural contrast, serve the salad with bread.
Bring a medium saucepan of water to a boil. Add lentils, garlic, and bay leaf. Simmer for 20 minutes, or until lentils are tender. Drain, and rinse lentils under cold water. Discard garlic and bay leaf. In a large bowl combine lentils, celery, onion, pepper, parsley, and rosemary.
In a small bowl, whisk together lemon juice and olive oil. Drizzle over lentil mixture, add feta, and stir gently to incorporate. Season with salt and pepper. Serve on a bed of baby lettuce, and garnish with cucumber slices.