Scoop a hollow in the centre of each onion half with a teaspoon, reserving the scooped out onion. Place the halves in a saucepan and cover with water. Add a little salt, bring to the boil and cook for about 15 minutes, until almost tender. Drain well.

Meanwhile, chop the reserved onion. Melt the butter in a non-stick frying pan and cook the bacon and onion for 4-5 minutes. Add the tomatoes and mushrooms and cook for a further 2-3 minutes.

Remove the pan from the heat, stir in the parsley and cheese and season to taste with salt, pepper and oregano.

Fill the onion halves with this mixture and put in an ovenproof dish. Cover the base of the dish with water to a depth of 1 cm (1/2 inch). Cook in the oven for 12 - 15 minutes.

Check the onions for tenderness after they have been boiled for about 15 minutes. The time will, of course, depend on how much onion you have

scooped out from the centres.