Prepare the kidneys by removing any membrane covering them, if present. Cut them in half lengthwise and cut around the fatty white core to remove it. Cut each kidney half into thick slices.
1 ½ lbs lamb kidneys
Heat a stockpot over medium heat, add the cooking fat and cook the onions, occasionally stirring, until they start to soften, about 6 minutes.
2 tablespoon cooking fat, 2 onions
Add the kidney slices to the hot pot and brown them on each side.
Pour in the stock and add the whole peppercorns, thyme sprigs, and bay leaf.
10 whole black peppercorns, 2 large sprigs of thyme, 1 bay leaf, 8 cups beef or lamb stock
Bring to a boil, then reduce to a simmer and let simmer, covered, for about 3 hours.
Add the carrot slices in the last 45 minutes of cooking.
2 carrots
When the soup is ready, discard the thyme sprigs and the bay leaf, and use a slotted spoon to remove most of the whole peppercorns.
1 bay leaf
Add 1 or 2 tablespoons of lemon juice to taste, taste for seasoning, and enjoy.
1 or 2 tablespoon lemon juice, Sea salt and freshly ground black pepper to taste