Shallow fry the chicken breasts in a little olive oil until browned and cooked through. Set aside. You can also cook them in tandem with the risotto later. Up to you.
In a saucepan, heat stock until boiling. Turn down to a simmer. (You can add extra stock down the track and heat to boiling if you need it. Depends how fast it simmers and best to have a little extra handy).
In another, larger saucepan, fry onion in olive oil over medium heat. The rice will quadruple or more in size. Add in garlic and fry till fragrant.
Add rice and, continuously stirring to ensure it doesn't stick, coat it with the oil and onion. You can add a little extra oil or butter in at this point. Keep the heat at medium but better to take it slow, than heat too fast. Continue to stir the rice for a few minutes until it starts to become translucent. You'll hear the rice make a popping noise as you cook.
Add in a ladle of the stock, and continue to stir as the rice soaks up the liquid. Judge the heat if cooking too fast. Some people add in a glass of wine before the first ladle of stock, but I don't think it's necessary.
Continue the add the stock, ladle by ladle, and keep stirring, ensuring the mix doesn't dry out.
Take your time. Have the glass of wine, instead.
The risotto will quadruple in size. Best way to tell if it's ready to your taste is to taste it. Taste it as you go along. I like it creamy, but not mushy, and certainly not chewy. When it tastes nearly done, but with a little bite to it, put in another ladle along with the peas and parmesan. Stir. Taste.
Add a final ladle so that the mix is liquidy. This will absorb off the heat and you don't want the rice to be dry.
You can either add the sliced chicken now to the rice to re-warm it, or top the serving later. Season to taste. Add herbs.