In a small mixing bowl, whisk together the honey, ketchup, soy sauce, and minced garlic until the sauce is smooth and well blended.
Arrange the boneless skinless chicken thighs in an even layer along the bottom of the slow cooker.
Pour the honey garlic sauce over the chicken, making sure each piece is coated. Cover the slow cooker with the lid.
Cook on High for about 3 hours or on Low for about 5 hours, until the chicken is fully cooked and reaches an internal temperature of 165°F.
Carefully transfer the chicken thighs to a serving dish and spoon some of the sauce from the slow cooker over the top.
Sprinkle with toasted sesame seeds, sliced green onions, and a small pinch of red pepper flakes before serving.