Sprinkle chicken with salt and pepper. This will ensure they are properly seasoned.
Heat a large skillet over medium-high heat. When the skillet is hot, add olive oil. Always start with a hot skillet before adding the oil. Do not add oil to a cold skillet, and then heat it.
Once the oil is hot (it only takes a few seconds), put your chicken thighs skin side down. Do not crowd the skillet, or they won’t brown properly. Skillet and oil must be hot to achieve a perfect sear on the skin and brown evenly.
Do not move the chicken thighs around. Let them sizzle in the hot oil for at least 3 minutes. Check on them just by picking them up with thongs. Once you are satisfied with the browning, remove them and put them into your crockpot in one single layer.
Make the ketchup: using a whisk, mix the tomato paste, water, white vinegar, Worcestershire sauce, maple syrup, and garlic until it is smooth and uniform. Normally, when making only ketchup, I don’t add the minced garlic, but since we are using it in this recipe, I added it to the ketchup to ensure it got well incorporated into the sauce.
Add the spices: garlic powder, onion powder, and mustard powder to the mix and whisk until smooth. This is your homemade ketchup.
Add the reduced-sodium soy sauce to the ketchup and mix well. Pour this sauce over the chicken thighs, making sure you cover them well.
Cook for 3 hours on HIGH or for 5 hours on LOW. It really makes no difference in the taste of the chicken if you cook it on high or low. It depends on what timing is more convenient for you.