In a mixing bowl, combine honey, soy sauce, ketchup, minced garlic, grated ginger, sesame oil, black pepper, and red pepper flakes (if using). Mix well to create the honey garlic sauce.
Place the boneless chicken thighs in the slow cooker, ensuring they are evenly spread out.
Pour the honey garlic sauce over the chicken, making sure all pieces are well-coated.
Cover the slow cooker and cook on low for 4 hours, or until the chicken is tender and cooked through.
Once done, carefully remove the chicken from the slow cooker and place it on a serving platter. You can shred it or leave it whole, depending on your preference.
If you like a thicker sauce, you can transfer the sauce to a saucepan and simmer it on medium heat for about 5-10 minutes until it thickens.
Drizzle the sauce over the chicken and garnish with sliced green onions.
Serve with cooked rice or vegetables.
For added flavor, consider marinating the chicken in the sauce for a couple of hours or overnight in the refrigerator before cooking.