Bring the stock to boil in a heavy based saucepan. Reduce the heat so the stock is simmering.
Melt the 50 grams of butter in another heavy based saucepan, add the crushed garlic clove and diced onion and sweat without colour for 2 minutes. Add the rice and seal for a further 2 minutes without colour.
Add a ladle of stock into the rice and stir. Continue to cook, stirring the rice continuously to prevent it from sticking. Reduce the heat so it simmers (too much heat will result in boiled rice) and continue to cook until the stock has been absorbed. Continue to add stock, one ladle at a time and cook the rice until the risotto softens. It will take between 18-20 minutes, depending on how much risotto you are making.
To finish the risotto, add the diced and chilled 20g butter and stir into the rice until fully incorporated. Add the parmesan and fold through the risotto. Season with salt and pepper to taste.
Add the sauteed mushrooms to the risotto and fold through. Divide the risotto between bowls and garnish with added Parmesan.