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Fry the onions in the butter for 15-20 mins until golden and softened. Butter a 900g loaf tin and put in the fridge to chill.
Heat oven to 200C/180C fan/gas 6. Combine all the ingredients in a large mixing bowl, and work everything together to form a large meatball. Remove the loaf tin from the fridge and transfer the mixture to it – it will look like a lot, but should all fit if you pack it in. Use a spatula to shape the top, so that it resembles a loaf of bread.
Cover with foil, then bake for 1 hr, or until the loaf reaches 70C on a meat thermometer. Heat grill to medium, then grill the loaf for 5-10 mins until golden on top. Leave to rest in the tin for 20 mins, then carefully tip the loaf onto a carving board or platter. Serve in slices with the usual trimmings, gravy and bread sauce.