Cook the lentils: In a medium saucepan, combine lentils, bay leaf, and water. Bring to a boil, then reduce heat, cover, and simmer 15-18 minutes until tender but not mushy. Drain and cool completely.
Make the dressing: Whisk together lemon juice, olive oil, salt, and pepper in a small bowl. Taste and adjust acid or oil balance as needed.
Chop herbs: Finely mince parsley and mint leaves with a sharp knife. A food processor can help, but avoid over-processing the herbs to retain texture.
Assemble the salad: In a large bowl, combine parsley mixture, cucumber, tomatoes, and red onion. Gently stir in cooled lentils.
Cool and season: Add the dressing and stir to fully incorporate. Chill for at least 30 minutes to let the flavors meld before serving.