Soak the mushrooms in 1 cup of water for 1-2 hours.
Remove mushrooms and slice, retain the water for use in the broth.
Fry the mushrooms and chopped onion together in a fry pan, add the bread crumbs and continue to fry for just a few minutes.
Put the meat slices between two pieces of plastic wrap and use a mallet to flatten each piece into about a 2- 3 inch wide and 4-5 inch long piece of beef.
Remove the wrap and season the meat with salt and pepper.
Put the mushroom and onion filling in a thin layer on each slice of meat and add 1 cornichon or sweet pickle and ¼ of a hard- boiled egg.
Roll the meat slices and secure with a skewer.
Fry the rolled beef in the pork fat over high heat to sear on all sides.
Pour in the water that you retained from soaking the mushrooms and stew on low heat for about 30 minutes.
Remove the beef rolls and thicken the sauce with a little flour, add back in the zrazy and cook for another 10- 15 minutes on low heat.
Remove the skewers and serve with stewed vegetables and Draniki.