Grease a large baking dish with butter. Preheat oven to 180ºC (heat above and below, no fan for now).

Pour the stock and the wine in a medium pot and bring to a boil.

Meanwhile, cut the ends of each onion, slice in half. Remove the heart from each side, keep in an air-tight container to use at another time. Carefully, remove each onion layer, one by one. Once the stock is boiling, add the onion layers and let them simmer for 7-10 minutes or until they've softened and become tendre. Do not fill the pot with onion layers, cook them in two batches, if necessary. Pick them out one by one and place them on a plate, to cool down. Reserve the stock for later.

Let's prepare the stuffing: grate the tomatoes and discard the skin. Add to a large bowl, along with crumbled feta, breadcrumbs, chopped parsley and basil, chopped spring onions, olive oil, salt and black pepper. Using a spoon, mix and press well, until you get a sticky doughy consistency.

Once the onion layers have cooled down, using that same spoon, fill each one with stuffing, generously. With your fingers, bring the seams of the onion together, as if it was a large fat cigar. Place on the baking tray (seam facing down). Repeat until you've stuffed all onions.

Pour some of the remaining stock in the baking tray, enough to cover the bottom of the tray, about 0.3-0.5cm high. Bake for 45-50 mins or until most onions are golden, and some have dark brown bubbles on the surface. You should notice the stock boiling halfway through the baking time, if this doesn't happen, bring up the temperature to 220ºC and bring the oven to fan mode.

Serve hot, with a simple side salad of greens.