Heat the vegetable oil in a large, heavy-based saucepan over a low-medium heat and add the chopped onion and cook gently until softened for about 10 minutes.
Place the stock in a separate saucepan over a low heat to keep warm.
Add the garlic to the saucepan with the onions and cook for a further minute then add the risotto rice and stir, coating it in the onion. Cook for about a minute then pour in the white wine. Stir and cook until the wine has absorbed into the rice and season to taste.
Add a ladle of warm stock to the rice and stir. Place the lid on and cook for a couple of minutes until absorbed. Continue this until the rice is almost cooked (it'll take about 20 minutes).
Stir the frozen peas into the risotto to cook them through for a couple of minutes. Add the Brussels sprouts and ham to heat through then add a little more stock or until the risotto is just cooked but still moist. Stir through the parmesan and sprinkle some on top to serve.