Put the chicken thighs in a bowl and marinate with spice paste, minced garlic and yoghurt for at least 15mins or overnight. Next, peel and chop the sweet potato into cubes. Place in a bowl of water with a squeeze of lemon juice to prevent discoloring. Heat the flora cuisine in a non-stick frying pan. Add onion and fry, stirring until softened. Add in chicken thighs with the marinade and fry, stirring for 5 minutes. Add the chopped tomatoes to the frying chicken and cook for 15minutes, stirring now and then. Meanwhile, put the chopped sweet potatoes in a pot with water and bring to the boil and cook till tender. Drain. Add the drained sweet potato to the chicken curry, mix in and heat through, add the nigella seeds and cream. Serve with naan bread and rice.