Preheat the oven to 200°C (Gas Mark 6 / 400°F).
Place the potatoes on a microwave-safe plate, and prick the skins a few times with a fork. Microwave for around 10-15 minutes, turning them over every few minutes, until just soft in the middle.
Meanwhile, heat the oil in a frying pan, and add the diced onion and garlic. Cook for a few minutes over a medium heat, then add the diced tomatoes. Turn the heat up a touch, and continue to cook for a further 5-10 minutes, until the tomatoes have broken down (squash them with the back of the spoon to help them along). Add the passata and plenty of salt and pepper, and leave to simmer.
To prepare the potato stuffing, add the chopped spring onions to a large bowl, along with the ricotta cheese, chopped spinach, and fresh basil. Add another pinch of salt and pepper, and mix thoroughly.
When the potatoes are cooked through, cut them in half, and scoop a small well in the centre of each half (you don't need to remove a lot of the flesh, just enough to give a dip for the stuffing to sit in). Divide the spinach and ricotta mixture evenly onto the six potato halves, and place them on a lightly greased baking tray.
Bake the potatoes for 10 minutes (or until they're as crispy as you'd like them), and serve on the tomato sauce.