Preheat the oven to 200 celcius.
Saute the onions in the olive oil in a large ovenproof casserole dish (use one with a lid) over medium heat.
When they are translucent, add the rice and stir for a minute.
Then add the stock and canned tomatoes.
Bring to a simmer, then add the zucchini (sliced or chopped) and stir to combine.
Sprinkle the parmesan over the top, cover and bake in the oven for 30 minutes.
Then remove the cover and bake for a further 7-10 minutes.