Simmer lentils in unsalted water according to package directions until just tender (about 15 minutes). Drain and set aside.
Meanwhile, heat olive oil in a large skillet over medium-high heat. Add in bell pepper and onion and saute until softened, about 5 - 6 minutes.
Add in carrots, garlic and ginger and saute 1 minute longer.
Add in drained lentils, hoisin sauce, soy sauce, rice vinegar, sesame oil and water chestnuts, cook while tossing, 1 minute (thin as needed with a few tablespoons of water. The lentils will absorb some of the sauce as the mixtures rests so you can thin with a little water as desired).
Toss in walnuts, sprinkle with green onions, cilantro and optional sesame seeds. Serve filling warm in lettuce leaves.