Heat a large skillet over medium heat. Add chicken. Cook and stir until chicken is broken into little pieces, about 5 minutes. Add red onion and scallions. Continue to cook and break up the chicken until browned, about 2 minutes. Add cilantro, mint, ginger, lemon grass, fish sauce, and red pepper flakes. Cook and stir until flavors blend, about 3 minutes more. Season with sea salt.