Halve the first onion and cut it into thin wedge slices. Quarter each pickle and set aside.

Using a meat mallet or the flat side of a heavy pan, pound each steak until it is about pancake-thin. Season both sides with salt and pepper, then spread roughly 1 tablespoon of brown mustard over one side of each steak.

Trim the smoked bacon into strips that match the length of each steak. Place two bacon strips along the center of each steak, then top with the quartered pickles and some of the sliced onion.

Fold the steak edges over the filling to form a tight roll. Tuck one end over the other and secure the seams with toothpicks as needed.

Heat a thin layer of grease in a large skillet over medium-high heat. Add the rolls in a single layer and brown them on all sides until nicely caramelized.

Remove the browned rolls to a plate. In the same pan, add the remaining 1 1/2 sliced onions and sauté for about 5 minutes until softened and translucent.

Return the rolls to the pan, pour in the stock, and add bay leaves, whole peppercorns, allspice and dried mushrooms if using. Reduce the heat to low, cover, and simmer gently for about 45 minutes, until the meat is tender.

Remove the rolls and whole spices from the pan. Purée the cooking liquid and onions with an immersion blender until smooth to form a gravy.

Whisk the flour with the cold water until smooth to make a slurry. Stir the slurry into the blended sauce, bring to a brief boil to thicken, and adjust seasoning with salt as needed.

Return the beef rolls to the sauce to rewarm briefly, remove toothpicks, and serve the zrazy topped with the rich gravy.