Preheat the oven to 180C.
Place the rice in a pan with two cups of water and bring to the boil. Reduce the heat to simmer, cover the pan and cook for 30 minutes.
Cut across the tops of the capsicums about 3cm from the top and remove the inner stalks and seeds. Replace the tops and set aside.
Heat the olive oil in a pan and saute the leeks with the garlic and cook for a few minutes. Add the chicken mince and saute for a few for a further 3-4 minutes. Add the stock and mushrooms and cook for a further 8-10 minutes or until the chicken is completely cooked. Add the cooked rice, pine nuts coriander and pepper and stir through. Stuff the capsicums and place in the oven to bake for 30 minutes.