Heat the oil in a large, heavy-based saucepan over a medium heat. Add the onion and cook gently for 10 minutes until softened. Place the stock in a saucepan over a low heat to keep warm.
Add the garlic to the onion and cook for a further minute, then add the rice and stir, coating it in the onion. Cook for another minute then add a ladleful of the stock. Stir and cook until absorbed into the rice, then add another ladleful.
Place the lid on and cook for a couple of minutes until absorbed. Continue this for another 20 minutes until the rice is almost cooked, then stir through the peas and ham. Cook them through for a couple of minutes then add a little more stock until the risotto is just cooked but still moist.
Stir through the parmesan and mustard, reserving a little to sprinkle on top for garnish.