Remove skin and gristle from kidney.
Cut across the kidneys and secure in an open grilling position on skewers.
Season with salt and pepper on both sides.
Brush with fat and grill for approx. 5 min on each side.
Meanwhile make a liquid paste with the mustard , Worcester sauce and vinegar.
Brush it liberally on both sides of the kidney.
Complete the cooking approx. 2 min on both sides.
Serve on trimmed, buttered rectangles of toast.