Heat the oven to 375 degree F.. Put the diced celeriac on a baking tray. Season and drizzle over a little vegetable oil. Roast for 15 minutes, or until tender and brown. Meanwhile, put the garlic, rosemary, and 100ml of olive oil in a small pan (so that the garlic is submerged, add more oil if you need to) and gently heat for 10 minutes, or until the garlic is soft and lightly golden. Remove, and cool the oil. You can use the leftover garlic oil for cooking, but keep it in the fridge and use within a week
Fry the shallot, leek and thyme with 50g of butter and 50ml olive oil. Season. When the vegetables are soft add the rice and stir until all the grains are coated. Gently heat for 1 minute to crack the rice (this allows for easier absorption).
Add 500ml of stock to the risotto and stir until it is all absorbed. Repeat another 2 times. This should take around 20 minutes. Add more stock if you need to, to get a creamy consistency. Take off the heat when the rice is tender, add the celeriac, the rest of the butter, cheese, and the parsley, and season with salt and pepper. Enjoy hot.