Heat the olive oil to medium-high in a cast iron (or regular non-stick) skillet. Add the onion and saute, stirring occasionally, until it begins turning brown, about 8 to 10 minutes. Transfer the sauteed onion to your crock pot.

Using the same skillet you used to cook the oven, place the chicken thighs skin-side down on the hot skillet and cook until browned, about 3 to 5 minutes. Flip using tongs and cook an additional 2 to 3 minutes on the other side - don't cook the chicken through.

Transfer the chicken thighs to the crock pot along with the remaining ingredients, except for the kale. Cover and turn on the crock pot. Cook for 4 hours on high or 6 hours on low.

Remove the chicken thighs from the crock pot using tongs and place on a cutting board. When cool enough to handle, discard the skins and bones and chop the meat. Place the meat back in the crock pot along with the chopped kale. Stir, cover, and allow soup to sit until kale has wilted, about 10 minutes. Serve heaping bowls with fresh lemon wedges, a drizzle of olive oil, and a sprinkle of red pepper flakes.