Place lentils in a large saucepan; cover with water to 2 inches above lentils, and bring to a boil. Cover, reduce heat, and simmer 20 minutes or until tender. Drain well, and set aside.
Combine lemon juice and next 5 ingredients (lemon juice through garlic) in a medium bowl; stir with a wire whisk until blended. Add lentils, tomatoes, cucumber, cheese, and celery to lemon juice mixture; toss gently to coat. Serve on lettuce-lined plates, if desired.