Remove the membranes from the kidneys. Using a sharp knife, slice them in half lengthwise. Cut out the white cores of the kidneys with my favorite game shears or kitchen scissors. Soak the kidneys in salt water for a full 12 hours in the refrigerator, rinse off, then soak in milk for another full 12 hours in the refrigerator.
Rinse off the kidneys, cut them into bite-size chunks, and pat them dry with a paper towel.
Preheat the oven to 425 degrees F.
In a mixing bowl, add the flour, salt, pepper, and smoked paprika and mix well with a fork. Add the kidneys and toss them until well coated.
Add the oil to a large cast iron skillet over medium high heat. Add the sliced onion and mushrooms and fry for about 5 minutes, or until soft and slightly golden-brown, stirring often.
Shake off any excess flour from the kidneys and add them to the pan. Continue to cook over a medium-high heat for about 3 minutes, turning occasionally.
Add the mustard, tomato paste, Worcestershire sauce and 1 cup of water and stir well to combine. Bring to the boil, reduce the heat to low, and simmer gently for about 10 minutes, or until the kidneys are tender and the sauce is thickened, stirring occasionally. Taste and season with more salt, pepper, and smoked paprika if desired.
Meanwhile, bake the Texas Toast according to package directions.
Remove the Texas Toast from the oven and place one slice on each of four plates. Divide kidneys and sauce evenly over each piece of toast and sprinkle each with a little bit of the parsley. Serve immediately.