Slowcooker Barbecue Pulled Chicken

Source: Elly Says Opa!

Slowcooker Barbecue Pulled Chicken
Heavily adapted from EatingWell / much less heavily adapted from Apple a Day

1 (14 oz.) can tomato sauce
3 chipotles in adobo, finely minced, and 2 tsp. adobo sauce
1/4 cup cider vinegar
3 to 4 Tbsp. honey
1.5 Tbsp. smoked paprika
2 Tbsp. tomato paste
2 Tbsp. Worcestershire sauce
1 Tbsp. dry mustard
1 tsp. kosher salt
1/2 tsp. cayenne pepper
2 tsp. aleppo chili powder
3 lbs. boneless/skinless chicken thighs and breasts (I used mostly breasts, plus 2 bone-in thighs I had & skinned)
1 large onion, thinly sliced in half circles
4 cloves garlic, minced

Whisk together the first 9 ingredients (through aleppo chili powder). Spray your slowcooker lightly with oil and then place the chicken inside. Season the chicken with salt and pepper. Scatter the onions and garlic evenly over the top, and then pour the barbecue sauce over the top. Cook on low for 5 hours (I just let mine go all day while I was at work, so closer to 10). Shred the chicken with two forks and, if desired, continue to cook for a little bit with the lid off to allow mixture to thicken.