Heat a little schmaltz in a large heavy based frying pan and
brown the meat all over in batches, keep to one side then repeat
with lamb kidneys if using.
Gently fry the onions in the same pan with a little schmaltz
and seasoning until golden.
Add Worcestershire sauce, pour in 500ml chicken stock and then
bring to the boil.
Thicken with a little cornflour mixed with cold water.
Stir in the lamb, kidneys and herbs then transfer to a
casserole dish.
Arrange the sliced potatoes on top of the meat and gravy,
drizzle with any excess butter leaving a little back to
finish.
Cover and place in the oven for about 1 hour 45 minutes or
until the potatoes and meat are both tender.
Remove the lid and brush the potatoes with a little more
butter, then turn the oven up to 210°C fan / 230°C (gas 8) and cook
until the potatoes become golden brown, or finish under the
grill.