Heat the olive oil in a large cast-iron skillet or sauté pan over medium-high heat. Season the chicken all over with salt and black pepper and cook the pieces until browned on all sides, about 7 minutes. Add the chorizo and continue cooking for another 2 minutes, until the chorizo has browned as well. Transfer the meat to the slow cooker. Pour the beer into the skillet, scraping the bottom to loosen any brown bits. Add to the slow cooker, along with the stock, vinegar, onion, bell pepper, garlic, paprika, cumin, and bay leaves and cook on low for 4 hours. If using spinach, add it 10 minutes before serving, giving it enough time to cook down in the warm braising liquid. Before serving, discard the bay leaves and taste and adjust the seasoning with salt and black pepper. Serve in wide shallow bowls with a ladle of the braising liquid poured over the top.