Heat the olive oil over medium-high heat in a medium-sized saucepan. Add in the onion, green pepper, and garlic, and cook until the veggies are just slightly tender, about 5 minutes.
Add in the lentils, salt, and water. Bring to a boil, reduce heat, and simmer for 12-15 minutes, or until most of the water is absorbed and the lentils are tender.
Add in the ketchup, Worcestershire sauce, maple syrup, and apple cider vinegar. Simmer until just thick and heated through, about 3 minutes. Serve immediately on top of whole wheat hamburger buns.