Grate the mushrooms for the risotto
Heat the chicken stock. This can be done on the stovetop or in a microwave. You want the stock to be hot but not quite boiling.
Heat the oven to 180 degrees celsius or 350 Fahrenheit.
Use a large oven-proof dish that has a lid. Start by heating the oil on the stovetop. Add diced onion, garlic and thyme sauté until softened and fragrant.
Add the arborio rice, cook for 1-2 minutes, stirring so that the rice does not catch on the bottom of the dish.
Add the grated mushrooms and cook with the rice for a minute or so.
Add the stock you heated earlier and bring it to a boil.
Cover with a lid and bake in an oven at 180 degrees celsius or 350 Fahrenheit for 20 mins.
Place whole portabello mushrooms in a roasting pan, gills facing up, drizzle with oil and balsamic vinegar. Season with salt and pepper and bake in the oven with the risotto until the mushrooms are tender for 10-15 minutes.
Serve the risotto topped with thickly sliced balsamic roast mushrooms, a little additional thyme and parmesan.