Remove kidneys from packaging and pat dry with kitchen paper, then slice in half removing any white core that's intact
Toss in the flour and season well with salt and pepper.
Melt the butter in a large non-stick frying pan, add the onion and sugar & sweat gently for 3-4 minutes, or until soft, sweet & tender & keep to one side.
Shake off any excess flour from the kidneys & saute them over a medium-high heat for 2-3 minutes then keep to one side.
Add the vinegar, tomato purée and mustard to the pan, then gradually add 250ml of light chicken stock or water, stirring constantly & bring to the boil; add the Worcestershire sauce to the pan and season with salt and freshly ground black pepper.
Reduce the heat and simmer gently for 5 minutes, then add the onions & kidneys to the sauce & simmer for a couple of minutes, at this point do not boil.
While the kidneys are cooking, grill the pancetta until nice and crisp then toast the bread and spread with butter and put on four warm plates.
Spoon the kidneys and sauce over the buttered toast and scatter with chopped chives, finish with crispy pancetta.
