If your lamb kidneys still have the membrane on them - kind of a skin coating - slice through it and peel it off. Note: This is generally not applicable for store bought kidneys, but something you’ll have to deal with if you’re butchering them yourself.

Using a sharp knife, trim any visible fat / connective tissues from the outside of each kidney. Slice each kidney in half; and trim away all of the white bits from the inside. Discard white bits, cut the kidney meat into ½-3/4" chunks.

Place prepared kidney meat in a strainer, rinse well for 2 minutes. Drain, set in a bowl, cover with milk. Cover with plastic wrap, set it in the fridge while you start the stew:

Trim roast of excess fat, cut into 1" cubes. Allow to come up to room temperature.

Pat beef cubes dry with a paper towel, season well with salt and pepper.

Measure about 1 tablespoon of the vegetable oil into a large, heavy bottomed pot. Heat over high heat.

Working in batches - and allowing space between each piece - brown the beef in the heated oil.

As beef is well browned on each side, remove from the pan and continue with remaining beef pieces, using more oil if necessary.

Remove all beef from the pan.

Add a splash of the beef broth to the pan, scraping and stirring the beef drippings - this is called deglazing.

Once the beef is all scraped off the bottom of the pan and incorporated, add red wine vinegar and tomato paste, stir well to combine. Add remaining beef broth, bay leaves, and the beef.

Bring to a boil, then reduce heat to a low simmer. Simmer over low heat for 1 ½ hours, or until meat is very tender.

As the beef cooks, prepare your vegetables: Quarter or slice the mushrooms, chop the onion, peel and slice the carrot, chop the celery, press or mince the garlic.

Add all of the vegetables to the pot, stir to combine.

Bring it to a boil again, then turn heat back down to low and simmer for another 30 minutes or so, until the vegetables are tender.

As the vegetables are boiling, drain the kidneys, rinse with cool water. Pat dry with paper towels, season with salt and pepper.

Add a little vegetable oil to a nonstick pan, and brown the kidneys over medium high heat - this should only take a couple of minutes. When browned all over, remove from heat, set aside.

In a small bowl, combine the dried parsley, gelatin, xanthan gum, and dried thyme. Be sure to stir well - you want the xanthan gum well distributed over the other ingredients, to prevent clumping.

Add herb mixture to the pot, stir well to combine. Add the browned kidney meat, stir again to combine.

Allow stew to simmer for a 5 minutes, to thicken the stew and cook the kidneys through.