Heat the oven to 180C/fan 160C/gas 4. 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.