Heat 1 tablespoon of oil in a large frying pan, add the onion and cook for 5-10 minutes until starting to soften. Add the garlic and risotto rice for the last minute.
Meanwhile, in a separate frying pan, heat the remaining 1 tablespoon olive oil, then add the chicken and mushrooms. Cook for 5-10 minutes over a medium heat, until the chicken is cooked through and the mushrooms are brown.
Add the stock to the rice pan, a ladleful at a time, waiting until almost all of the stock has been absorbed before adding the next ladle. This should take about 20 minutes.
Stir through the peas and cook for a further 2-3 minutes. Just before serving, stir through the chicken, mushrooms and mozzarella. Season to taste and serve.