Preheat the oven to 200°C / 400°F, or 180°C / 350°F fan-forced.
Heat the extra-virgin olive oil in a large oven-safe frying pan over medium heat.
Add the finely diced onion, freshly minced garlic, roughly diced mushrooms, and sea salt flakes.
Cook for 3 to 4 minutes, stirring occasionally, until the onion has softened and the mushrooms have released some moisture.
Add the arborio rice to the pan.
Stir for 1 to 2 minutes, or until the rice grains begin to turn from white to slightly translucent.
Add the white wine and stir for 1 to 2 minutes, or until the liquid has mostly evaporated.
Add the chicken stock or vegetable stock.
Stir until fully combined and bring the mixture to a simmer.
Cover the pan with a lid. If the pan is not oven-safe, transfer the mixture to a deep baking tray and cover tightly with a double layer of foil.
Transfer to the oven and bake for 20 minutes, or until the rice is tender and the liquid has mostly absorbed.
Remove the risotto from the oven.
Stir through the freshly grated parmesan and unsalted butter until fully incorporated.
Season to taste with additional sea salt flakes and freshly cracked black pepper.
Allow the risotto to cool completely.
Transfer the risotto to the refrigerator for at least 1 hour, or preferably overnight, until fully chilled and firm enough to shape.