In a medium mixing bowl, stir together the pork, bell pepper, ginger, garlic, sweet chili sauce, fish sauce, and sesame oil until thoroughly combined. Place a large skillet over medium heat. Swirl about 2 teaspoons of vegetable oil in the pan. Add the pork mixture to the pan, breaking up the meat with a spatula, and cook until browned and done. Stir in the water chestnuts, and oyster sauce until coated. Remove the mixture from the heat. Sprinkle with green onions and cilantro. Spoon the pork mixture into the lettuce cups. Serve with Sriracha on the side. Enjoy promptly.