My mother used to make kidney stew. She would add a bay leaf. Plus, I was taught to cook the kidneys as little as possible, or they'll get tough, certainly not for an hour or more. I just drop the kidney in boiling water for about 5 minutes, take it out and cook the diced potatoes separately, adding the cut up kidney about 5 minutes before potatoes are done, then add flour mixed with a little water kidney was cooked in and allow stew to thicken. Add a dash of sherry and serve.