Slice them, removing the white "core" as far as practical. Fry one chopped onion in lard until softening. Have ready some thick (double, heavy) cream and Dijon mustard plus salt and pepper.
Add kidneys to pan and fry, stirring occasionally, for just 3 or 4 minutes then add about 2 teaspoons mustard and enough cream to make a sauce. Stir, add S&P to taste and serve.
It is important not to overcook the kidneys - under cook rather than over! Bleeding slightly i the middle is just fine.