Mild chicken curry

Name: Mild chicken curry
Ingredients: 1-1½ tsp coconut oil (we used Fushi) or sunflower oil; 1 large onion, finely chopped; 2 fat garlic cloves, crushed; 1cm fresh ginger, grated or finely chopped; 1 tsp ground coriander; 1 tsp yellow mustard seed; 1 tsp garam masala; ½ tsp ground cumin; 1 x 500g pack chicken pieces (thighs and drumsticks), or thighs; 1 chicken stock cube; 1 cinnamon stick; 250g Greek yogurt, at room temperature; 2 tbsp sultana; handful chopped coriander, to serve (optional)