Preheat the oven to 180°C. Cook the onions in boiling water for 10 minutes, then drain well and rinse under cold water until cool enough to handle. Using a sharp knife, cut a thin slice from the top and remove the centre from each onion, making a hollow about 4 cm deep and 5 cm wide.
Chop the onion centres and tops ﬁnely and combine the chopped onions in a bowl with the cheese, nuts, raisins and sage. Season to taste. Pack the mixture into the centre of each onion, pressing it down ﬁrmly. Pile any leftover ﬁlling on top.
Arrange the stuffed onions in a shallow, heatproof dish that is large enough to hold them comfortably. Pour the vegetable stock over the onions. Brush the onions with the melted butter and sprinkle the light brown sugar over the top.
Cover the dish with foil, but do not let the foil touch the onions. Bake in the oven for 15 minutes, then remove the foil and bake for a further 20–30 minutes, or until the onions are lightly browned and tender and the top of the stuffing is crisp.
Remove the dish from the oven, baste the onions with the cooking juices and serve.
Ham-stuffed Onions: Prepare as for stuffed onions, but replace the cheese mixture with 200 g minced ham, mixed with a little sour cream and chopped celery. Serve with rice as a main course, or on their own as a light lunch.