Start by rinsing the red lentils in a fine mesh strainer. Place them in a pot and add 2 ½ cups of water. Bring to a boil, then reduce the heat and let them simmer for about 15-20 minutes, or until tender. Drain any excess liquid and set aside.
In a large skillet, heat the olive oil over medium heat. Add the chopped onion and sauté for 2-3 minutes until it becomes translucent. Next, add the red bell pepper, matchstick carrots, minced garlic, and ginger. Sauté for an additional 5 minutes until the vegetables are tender-crisp.
Stir in the cooked lentils, hoisin sauce, soy sauce, rice vinegar, and sesame oil. Mix well to combine all the flavors. Add the chopped water chestnuts and walnuts, and cook for another 2-3 minutes until everything is heated through.
While the filling is cooking, prepare your lettuce leaves. Carefully separate the leaves from the head of lettuce, ensuring they remain intact for wrapping. Rinse them under cold water and pat them dry.
Spoon the lentil mixture into the center of each lettuce leaf. Top with chopped green onions and cilantro. If desired, sprinkle with sesame seeds for an extra touch. Serve immediately and enjoy your Asian Lentil Lettuce Wraps!