Heat the olive oil in a cast iron skillet or other heavy-bottomed pan over medium-high heat. Season the chicken thighs with salt and pepper. Place them skin-side down in the hot oil and sear until the skin is crisp and golden brown.
Transfer the seared chicken thighs to a crockpot, arranging them skin-side up in an even layer.
In a medium mixing bowl, combine the ketchup, soy sauce, honey, minced garlic, onion powder, dried thyme, dried parsley, and dried mustard. Whisk until smooth and well-blended.
Pour the prepared sauce over the chicken thighs in the crockpot. Cover with the lid and cook on HIGH for 3 hours or on LOW for 5 hours, or until the chicken is fully cooked and tender, and the sauce has thickened.
Once cooked, remove the chicken from the crockpot and transfer to a serving dish. Spoon the sauce from the crockpot generously over the chicken and garnish with freshly chopped parsley, if desired.