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Snap off the thick end of the celery sticks and pull off any stringy bits. Cut them so they fit into the base of a 30 x 20cm ovenproof dish. Bring a large pan of salted water to the boil. Cook the celery for 6-8 mins until just tender. Drain, then set aside.
Heat the oil in the same pan over a medium-low heat. Cook the onion for 8-10 mins until soft. Stir in the garlic and thyme. Cook for 1-2 mins until the garlic is soft but not brown.
Turn the heat up to medium and add the tomatoes, tomato purée, vinegar and sugar. Season, then cook for 10-15 mins until the sauce has thickened slightly.
Meanwhile, heat the oven to 200C/180C fan/gas 6. Arrange the celery in a single layer in the dish. Dot over a quarter of the mozzarella. Combine the breadcrumbs and parmesan in a small bowl. Set aside.
Pour the sauce over the celery. Top with the rest of the mozzarella pieces. Scatter over the breadcrumb mixture and bake for 20-25 mins until piping hot and golden on top.