Heat 3 tablespoons of the oil in a saucepan with one of the garlic cloves. When you can smell the garlic, add the tomato passata and 250 ml (1 cup) water. Season with salt and pepper and bring to the boil. Lower the heat and simmer for about 15 minutes or until it is cooked and still rather liquid.
Preheat your oven to 180°C (350°F/Gas 4) and bring a saucepan of lightly salted water to the boil. Cut off the stalk from each eggplant and then cut it in half lengthways. Scoop out the flesh with a tablespoon (a pointed one is easiest), so that you have a boat-shaped, thin shell. Chop up the flesh and keep it on one side. Boil the eggplant boats for about 10 minutes in the pan of water, testing them with a fork. They should be soft, but still in shape. Drain.
Meanwhile, heat the remaining oil and garlic in a large non-stick saucepan. Add the eggplant flesh, celery and paprika and sauté over medium heat until it is softened and very lightly golden. Add the minced meat and continue cooking for 10–15 minutes, until the meat is lightly golden, seasoning lightly with salt. Break up any clusters of meat with a wooden spoon. Mix in 250 ml (1 cup) of the cooked tomato sauce, cook for 2–3 minutes more and then remove from the heat to cool slightly. If your pan is big enough, mix in the parmesan, feta, and parsley — if not, mix it all together in a larger bowl. Check the seasoning. Remove the garlic clove from the tomato sauce.
Ladle tomato sauce onto the bottom of a baking dish to a depth of 2 cm (¾ inch). Use a dish that is large enough to fit all the eggplants fairly compactly in a single layer. Fill the eggplant boats with meat filling, patting it in well with your hands, but don’t make it too compact. Arrange the eggplants on top of the tomato sauce and splash the tops with the rest of the tomato sauce, but don’t drown them.
Bake for 30–45 minutes (or longer if necessary) until the sauce is bubbling up and a little bit of a golden crust has formed on the top in places. Remove from the oven and cool slightly before serving. These are also good served at room temperature.