Marinate the chicken in yoghurt and spices for up to 6 hours.
Fry onion and garlic in a little oil,when soft add ginger,tomato paste and spices until fragrant,can skip this if in a hurry.Add to slowcooker with chicken(remove excess yoghurt) and diced tomatoes.
Cook for 6 hours on low.About 15 minutes before end of cooking add cream or milk,can add more garam masala if needed.Might need to thicken sauce with a little cornflour.
Serve with jasmine or basmati rice,and papadums.