Heat a large pot on medium heat. When the pan is hot, add the olive oil and red onion. Cook the onion for 2-3 minutes until the onion is slightly caramelised. Add the garlic and cook for another minute, making sure to stir regularly.
Add the mushroom and cook for about 5 minutes until it has reduced in size and let out some of the water.
Add the remaining ingredients to the pot and cover the pot with a lid. Reduce the heat to low and allow the risotto to simmer for 20-25 minutes. The rice will slowly by absorb all the liquid. Stir the risotto at least every five minutes to make sure that the rice doesn't stick to the bottom.
The risotto is finished once the rice has cooked well. Avoid overcooking the rice because it will become mushy. In the end, you want a soft but creamy consistency with a bit of liquid remaining. If the risotto looks dry, add some more vegetable stock. Do a final taste test to make sure that there is enough salt and pepper. Often the rice can soak up a lot of the flavour so be generous with the seasonings.