Crumble 1/2 piece naan into coarse crumbs; transfer to a 6-quart slow cooker. Add the red onion, potatoes, chile and cilantro stems. Arrange the chicken thighs in a single layer over the vegetables and sprinkle with the garam masala, 3/4 teaspoon salt and a few grinds of pepper. Dot with the butter and add the cinnamon stick. Pour the tomato sauce over the top. Cover and cook on low, 7 hours.
Uncover and switch to the warm setting or turn off. Gently stir in the heavy cream, breaking the chicken into large chunks; let stand 10 minutes. Stir in the cilantro leaves and season with salt. Serve with the remaining naan.