Heat oil in a large skillet over medium-high heat. Add onions and sauté until translucent, 3 to 4 minutes. Mix hoisin, soy sauce, brown sugar, vinegar, sesame oil, pepper flakes and 1/4 cup of water; cook until mixture thickens and darkens in color, 3 to 4 minutes. Stir in Lentil-Walnut Mix; cook to heat through a couple of minutes longer. Serve, letting each person spoon the mixture into the lettuce cups, topping with a squeeze of lime juice, hot sauce (like sriracha) and a sprinkling of scallions and peanuts.