Brown the mince: Pat the mince dry, then heat the oil in a large pan until almost smoking. Add the mince so it sizzles on contact and fry hard for 5 to 6 minutes until deeply browned. Don't crowd the pan.
Soften the veg: Add the onion and green pepper, cooking for 4 to 5 minutes in the beef fat until softened. Stir in the garlic for the last minute so it doesn't burn.
Cook out the purée: Stir in the tomato purée and fry it for 30 seconds, then add the passata, Worcestershire sauce, mustard, brown sugar and paprika. Season well.
Reduce it: Simmer for 8 to 10 minutes, stirring, until the sauce thickens and clings to the mince with no watery pool left in the pan.
Serve: Toast the buns cut side down, then spoon the mince generously onto the bases and add the lids. Serve straight away while hot.
