Rinse 1 cup of lentils under cold water, removing any debris. Combine lentils with 3 cups of water and a bay leaf in a pot. Bring to a boil, then simmer for about 20 minutes until tender but not mushy. Drain and let cool.

In a mixing bowl, whisk together 1/4 cup of fresh lemon juice and 1/3 cup of extra-virgin olive oil until well combined. Set aside.

Finely chop 1 cup of curly parsley and 1/2 cup of mint leaves. Dice 1 English cucumber and chop 2 medium tomatoes. Slice 1/4 of a red onion thinly. Combine all ingredients in a large mixing bowl.

Add cooled lentils to the bowl with herbs and vegetables. Gently fold to combine using a spatula.

Season with kosher salt and freshly ground black pepper to taste. If using, add sumac and toss gently to coat.

Let the salad sit for about 10 minutes before serving to allow flavors to meld.