Preheat the oven to 400F. Place the chilies in a bowl and pour in just enough hot water to cover. Set aside to soak for 30 minutes. Meanwhile, place the unpeeled garlic cloves on a cookie sheet and toast in the oven for about 10 minutes until soft. Remove from the oven and let cool.
Drain the chilies, reserving 1/2 cup of the soaking water. Seed the chilies, if you like, and chop coarsely. Place the chilies and reserved soaking water in a blender or food processor and process to a puree. Peel and mash the garlic in a bowl.
Place the chili puree, garlic, onion, and tomatoes in the slow cooker and stir in the stock. Season the chicken portions with salt and pepper and place them in the slow cooker. Cover and cook on low for about 5 hours until the chicken is tender and cooked through.
Lift the chicken out of the slow cooker with a slotted spoon, cover, and keep warm. Pour the cooking liquid into a pan and bring to a boil on the stove. Boil for 5-10 minutes until reduced. Place the chicken on warm plates, spoon the sauce over it, and serve.