For the vindaloo gravy, heat the oil in a wok or large frying pan. Add the cinnamon stick and garlic and gently fry until the garlic is translucent. Add the chopped onions and cook until light-brown. Add the vindaloo spice mix and gently fry for 2-3 minutes, then stir in the tomatoes, vinegar and palm sugar and cook for 3-5 minutes. Pour in the chicken stock and simmer for 5-6 minutes. Season, to taste, with salt and stir through the chopped coriander leaves. Keep warm until ready to serve.