Rinse the lentils under cold running water and place them in a medium saucepan with four cups of water and a pinch of salt. Bring to a boil, then reduce heat and simmer for 20-25 minutes until tender but firm.

Drain the lentils and rinse with cold water to stop the cooking process. While cooling, prepare the vegetables and herbs by finely dicing the red onion and cucumber, and chopping the parsley and mint.

In a small jar or bowl, whisk together olive oil, lemon juice, Dijon mustard, and minced garlic. Season with salt and pepper.

In a large mixing bowl, combine the cooled lentils, diced onion, cucumber, and chopped herbs. Pour the dressing over and toss gently to coat. Adjust seasoning if necessary.

Let the salad rest in the refrigerator for at least 30 minutes before serving to allow flavors to meld.