In a large pot, add the oil, lemongrass, kaffir lime leaves (if using), ginger slices, onions, and Thai chili paste on medium heat stirring for 3 minutes to infuse the flavors. Add the chicken slices to the pot and cook for about 5 minutes until they are cooked through. Stir in the minced garlic, fish sauce, soy sauce, lime juice, and nap a cabbage. Mix well.