Rinse lentils well, drain. Place in a pot and cover with 3–4” of water; bring to a boil; reduce to simmer. Check lentils for doneness after 15 minutes, but they should take about 20 minutes in total. You will know they are cooked if they still retain a slight tooth—al dente. Do not overcook. When the lentils are cooked, remove from heat, drain, and place under cold running water to stop the cooking process. Once cooled, place lentils and all vegetables in a large bowl and toss to combine. Whisk olive oil, lemon zest, lemon juice, salt, pepper, and sumac, and pour over salad. Toss 1 or 2 more times and serve.