Step 1: Remove the Membrane
Make a cut in the thin membrane surrounding the kidneys using a sharp knife. Remove and discard the membrane.
Step 2: Slice in Half
Slice the kidneys in half, lengthwise. Cut out and discard the round lump of white fat in the center of each kidney half.
Step 3: Cut Into Chunks
Rinse the beef kidneys thoroughly under cool, running water. Pat the meat dry with paper towels. Cut each beef kidney half into 1 1/2-inch chunks.
Step 4: Soak Your Beef
Fill a large bowl with cold water. Add a small amount of plain vinegar or lemon juice and a pinch of salt. Put the beef kidney pieces into the water and allow them to soak for two hours.
Step 5: Drain the Soaking Water
Drain the kidney chunks in a colander, discarding all of the soaking water and the liquid that comes out of the meat.
Step 6: Heat Some Liquids
Pour water and your choice of wine into a large saucepan. Plan on 1 cup of wine and about 1 3/4 cups water for every 3 to 3 1/2 pounds of raw beef kidneys. Bring the liquid to a boil.
Step 7: Add Meat and Veggies
Stir the kidneys into the liquid along with chopped vegetables such as mushrooms or potatoes, if desired, and seasonings like garlic or thyme. Allow the mixture to boil, then reduce the heat to low.
Step 8: Cook Until Tender
Put the pan's lid in place. Continue to cook on low, stirring only occasionally, until the beef kidneys and vegetables are tender, about 45 minutes.