Preheat the oven to 180 C. Place the celeriac, onions and garlic into a large roasting tin and drizzle over the olive oil. Toss to coat. Cover with foil and cook in the oven for 40-50 minutes, stirring a few times during cooking. Remove the foil 5-10 minutes before the vegetables are cooked so that they can brown. Remove from the oven and set aside to cool slightly.