Heat oven to 180C/fan 160C/gas 4. Rub 1 tbsp olive oil into the chicken thighs. Mix the garlic, cumin, coriander and paprika together, then rub all over the chicken thighs on both sides.
Heat a large flameproof casserole, add the chicken thighs and cook over a medium heat for 5 mins until golden on both sides. You might need to do this in 2 batches, depending on the size of the casserole. Set the chicken aside. Turn down the heat, add the remaining oil and fry the onion for 5 mins until softened.
Stir in the rest of the ingredients, apart from the mint and bring to the boil. Place the chicken thighs on top and pour in any juices. Cover and cook for 1½ hrs, until the meat is tender and the sauce thickened. Can be cooled and frozen at this stage for up to 1 month. Defrost thoroughly in the fridge, then gently warm through. Scatter with fresh mint leaves and serve with couscous or rice.