Heat the oil in a saucepan, add the onions and cook on a medium heat for 5 minutes. Add the tomatoes, garlic, sage and sugar, season with salt and pepper, then cover and cook gently for 30 minutes.
Peel the celeriac, cut into slices, then cut these into thick 5cm long matchsticks. Add to the tomato sauce with 4 tbsp water. Bring to the boil, cover and simmer gently for a further 20 minutes or until the celeriac is tender. Garnish with fresh sage.
For a sweeter celeriac dish, cut the vegetable into sticks, then cook for 15 minutes in boiling, salted water. Drain, then cook gently in a pan with 30g butter and 2 tbsp raisins for 5 minutes, stirring regularly.