Combine the pineapple juice, soy sauce, ketchup, ginger, garlic, and crushed red pepper flakes together in a zip lock bag. Mix well. Add the boneless skinless chicken thighs to the marinade; seal and place into the refrigerator for at least 4 hours.
Remove the chicken from the refrigerator 20 minutes prior to cooking. Place the chicken on a plate then season both sides with sea salt and freshly cracked pepper, to taste. Heat a grill pan coated in cooking spray over medium heat. Once the pan is hot, add the chicken thighs and cook for 4-5 minutes; flip then cook for an additional 4-5 minutes or until chicken is cooked through. Sprinkle the green onions over the cooked chicken and serve. Enjoy.