Pre-heat your oven to 200 °C.
Place an ovenproof pan or casserole dish on the stovetop over medium heat (if you don't have an appropriate dish, cook the chorizo and onions in a fry pan and then transfer to an ovenproof dish).
Cook the diced chorizo and sauté until golden and crisp. Remove chorizo but leave fat in the pan.
Add butter to the pan and cook the onions until translucent (about 3-4 minutes).
Add the rice and stir to coat the grains, around 1-2 minutes.
Add the remaining wet ingredients - the red wine, chopped tomatoes and chicken stock. Clamp on a lid and place in the oven for 30 minutes.
In the final 5 minutes, stir in the frozen peas and half of the parmesan. Return to the oven.
The risotto is done when the moisture is absorbed and the rice is al dente.
Serve with remaining parmesan and enjoy!