With a sharp knife, trim the strings from the celery stalks and cut into short lengths (about 4 cm/1½ inches). Bring a pan of salted water to the boil and cook the celery for about 20 minutes until it is soft but not collapsing. Preheat the oven to 200°C (400°F/Gas 6) and lightly oil an 18 cm (7 inch) square shallow ovenproof dish.
Heat the olive oil and garlic in a small pan over medium heat. When you can smell the garlic, add the passata and 3 tablespoons of water. Season with salt and pepper and add the basil. Simmer over low heat for about 10 minutes for the flavours to mingle and the sauce to thicken a little. Fish out the garlic clove and throw it away.
Drain the celery. Spread the flour on a plate and pat the celery pieces lightly into it to coat both sides. Spread about a third of the tomato sauce over the bottom of the dish. Arrange the celery on top, in a single layer or slightly overlapping. Dollop the rest of the tomato sauce on top. Bake for 20 minutes, or until the top is golden in places. Sprinkle with the parmesan and bake for another 10 minutes or so until it is golden and crusty. Leave to cool for a few minutes before serving.