Mix the cucumber and water together in a pitcher and refrigerate to infuse until serving.

Heat a pot over medium heat. Once hot, add the olive oil. Sauté the onion, carrot, celery, and garlic until they start to soften, about 5 minutes.

Stir in the chicken broth, diced tomato, thyme, and lentils. Bring to a simmer and cook for about 20 to 30 minutes, until the lentils are soft and the soup builds flavor. If needed, add more broth to reach the desired consistency.

Check the lentils for doneness; they should be tender but not mushy. Remove the thyme sprigs before serving.

Ladle the soup into bowls and serve hot. Arrange the cheese slices on a plate with the crackers. Serve the cucumber water as a refreshing beverage alongside the meal.