This is a tasty dish that is not too spicy. If you like more spice, feel free to add something like sriracha, cayenne pepper, or hot chile paste.
Add the red onion, potatoes, chile and cilantro stems to a slow cooker. 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.