Sprinkle chicken with salt and pepper.
Heat a large skillet over medium-high heat. Add the olive oil and place the chicken thighs, skin side down. Brown them for 3 minutes and turn. Brown them for 3 minutes on the other side. Place them inside the crockpot in a single layer.
Make the ketchup: whisk together the tomato paste, water, white vinegar, maple syrup, Worcestershire sauce, soy sauce, and minced garlic until smooth.
Add the spices: garlic powder, onion powder, dried parsley, and mustard powder to the mix and whisk until smooth.
Cook for 3 hours on HIGH or for 5 hours on LOW. Serve with more parsley on top for presentation.