Peel, halve and slice the onions.
Heat the oil in a pan and fry the onions over a low heat until slightly softened.
Meanwhile, peel the celery, coarsely grate and add to the pan. Season with salt and pepper and continue to cook for 3 minutes.
Bake the flatbread in a preheated oven at 150°C (130°C, gas 3) for 5 minutes.
Drain the mozzarella and coarsely crush the tomatoes.
Sprinkle the curry powder over the celery. Add the tomatoes, together with their liquid and simmer for 5 minutes over a medium heat.
Season the tomato ragu with salt, pepper and a pinch of sugar. Fold in the mozzarella. Cut the flatbread into strips and serve with the ragu.