Heat the sesame oil in a pan over medium heat. Add the onion, garlic & ginger to the pan and cook for 1-2 minutes, or until fragrant.
Add the beef crumbles, hoisin, soy sauce, rice vinegar, water chestnuts and serrano pepper to the pan. Stir to combine and cook until hot and bubbly. Stir in 1/2 of the green onions.
To serve, spoon the filling into each lettuce cup and top with more green onions, cashews and lime wedges.