In a medium bowl, combine the ketchup, apple cider vinegar, brown sugar, minced garlic, paprika, chili powder, Dijon mustard, cayenne pepper, salt, and black pepper. Whisk until the mixture is smooth.
Pour the water into the bottom of a 6-quart or larger slow cooker.
Arrange the sliced onions in an even layer at the bottom of the slow cooker.
Place the chicken leg quarters on top of the onions.
Pour the prepared sauce evenly over the chicken pieces, ensuring they are well coated.
Cover the slow cooker and cook on LOW for 6 to 7 hours, or on HIGH for 3 to 4 hours.
The chicken is done when it is tender and the internal temperature reaches 165°F (74°C) when checked with a meat thermometer.
Carefully remove the chicken from the slow cooker and serve hot, with extra sauce from the pot spooned over the top.