Place the veal (or chicken) slices between two pieces of parchment paper and use a meat mallet to pound them to about 1/4-inch thickness.

Season the pounded cutlets on both sides with kosher salt and coarse black pepper.

To build the stuffed cutlets, layer two of the meat slices with 2 slices of Swiss cheese, 2 halves of salami, and 3 to 4 sage leaves each.

Top with the remaining two meat slices, pressing down slightly to form two stuffed scallopini "sandwiches."

Place the all-purpose flour into a shallow bowl, and in a separate bowl, whisk the egg with 1 tablespoon of water.

Dredge each stuffed cutlet first into the flour (shaking off excess), then dip into the egg wash, and finally dredge once more in the flour to coat.

Heat the olive oil and 2 tablespoons of butter in a large skillet over medium-high heat until bubbling.

Carefully add the stuffed cutlets and cook until golden brown and crispy on one side, about 2 to 3 minutes.

Carefully flip and cook until golden on the other side, then transfer to a warm platter.

Add the diced shallot and sliced mushrooms to the remaining drippings in the skillet, cooking until the shallots soften and the mushrooms are browned, about 3 to 5 minutes.

Deglaze the pan with the Marsala wine, scraping up any browned bits from the bottom, and simmer until the liquid reduces slightly.

Pour in the beef broth and simmer until the sauce reduces and thickens enough to coat the back of a spoon.

Remove the pan from the heat and swirl in the remaining 2 tablespoons of cold butter to create a glossy, velvet emulsion.

Return the stuffed scallopini to the pan to briefly glaze in the hot sauce, then serve immediately topped with a shower of fresh parsley.