Cut chicken fillet into small pieces and fry on olive oil until ready. Set aside.
Cut onion into small pieces, fry most of it until slightly transparent for approximately 5 min (leave some onion to use later in rice). Add sliced mushrooms and fry until ready. Set aside.
In a frying pan cook remaining onion on olive oil for 3 min, add arborio rice, rosemary and mix everything. Add chicken stock and mix everything until rice absorbs the liquid. Continue adding boiled water and stirring for 20-25 min until rice is cooked al dente. Add salt and pepper to taste.