Heat the olive oil in a heavy-bottomed pan or cast-iron skillet. Season the chicken thighs with salt and pepper, then sear them skin-side down until the skin becomes golden and crispy. Once seared, transfer the chicken, skin side up, to the crock pot.
In a medium bowl, whisk together the ketchup, soy sauce, honey, minced garlic, onion powder, dried thyme, dried parsley, and dried mustard. Pour this sauce over the chicken in the crock pot. Cover with the lid and cook on HIGH for 3 hours or on LOW for 5 hours until the chicken is thoroughly cooked and the sauce has thickened.
When done, remove the chicken and arrange it on a serving dish. Spoon the sauce from the crock pot over the chicken, garnish with fresh parsley if desired, and serve warm.