Preheat the oven to 350°F Have ready a small, buttered oven-proof dish. Put the stock and white wine into a medium-sized saucepan, bring up to a boil, then turn down the heat to a simmer.
Meanwhile, trim 1/4 inch off the top and bottom of the onions, then cut them in half lengthways. Remove the skin and carefully take out most of the insides, keeping only two or three layers of the outer skin of each onion intact (set aside the insides for other uses). Carefully separate the outer layers from each other and place, a few at a time, in the simmering stock. Cook for three to four minutes, until just tender, then drain well and leave to cool slightly. Repeat until all the onion has been blanched.
To make the stuffing, use a coarse cheese grater to grate the tomatoes into a large bowl (you will be left with most of the skin in your hand; discard it). Add breadcrumbs, feta, parsley, oil, garlic, spring onion, salt and pepper. Fill each onion skin with stuffing. Pull the sides together so that you end up with a fat cigar shape. Place seam-side down in the buttered dish. Bake for 45-50 minutes, until soft and lightly coloured, with the stuffing bubbling. Serve warm with a drizzle of oil.