Tip the rice into a bowl. Cover with cold water. Whisk for 1-2 mins (you can use a fork) till the water goes cloudy. Drain the rice. Rinse it under cold water and tip it back into the bowl. Cover with cold water and set aside.
Peel and finely chop the onion. Peel and crush or grate the garlic. Finely chop the stalks from the parsley sprigs (keep the leaves for later). Roughly chop the olives.
Chop the chicken breast fillets into bite-size chunks. Warm a pan for 1 min over a medium heat. Add ½ tbsp olive oil and the chicken. Fry for 5-8 mins, turning the chicken once or twice, till it's golden brown. Lift the chicken out of the pan and pop it on a plate.
Add another ½ tbsp oil to the pan along with the onion and mushrooms. Season with a pinch of salt and pepper. Fry for 5 mins, stirring now and then, till the veg are a little browned and soft.
Stir the garlic, parsley stalks and olives into the pan. Tip the chicken back in. Pour in the chopped tomatoes, quarter fill the can with water (about 100ml) and add to the pan. Put on the lid and bring the stew to the boil. Turn the heat down and simmer for 10 mins.
While the chicken cooks, drain the rice and tip it into a small pan. Pour in 400ml boiling water. Add a pinch of salt. Cover. Bring to the boil. Turn the heat right down. Gently simmer for 6-8 mins till all the water has been absorbed. Take off the heat and set aside, lid on, for a few mins to finish cooking the rice.
Taste the chicken stew. Season with a pinch more salt or pepper if you think it needs it.
Divide the rice between 2 warm bowls. Top with the chicken stew. Garnish with the parsley leaves to serve.