Bring a medium saucepan of water to a boil. Add lentils, garlic, and bay leaf. Simmer for 20 minutes, or until the lentils are tender. Drain and rinse the lentils under cold water. Discard the garlic and bay leaf. In a large bowl combine lentils, celery, onion, pepper, parsley, and rosemary.
In a small bowl, whisk together the lemon juice and olive oil. Drizzle over lentil mixture, add the feta, and stir gently to combine. Season with salt and pepper. Serve on a bed of baby lettuce, and garnish with cucumber slices.
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