Season the veal (it doesn't need pounding―escalopes come that way) with salt and pepper and lightly dredge in flour, shaking off any excess.
In a large skillet, heat 2 tablespoons of oil over medium heat. Place 3 escalopes in the pan and cook until the edges turn golden brown, about 40 seconds. Turn over and cook for another 40 seconds. Transfer to a plate and cover loosely with foil to keep warm. Repeat with the other 3 escalopes, adding the remaining oil.
To make the sauce: Deglaze the pan by adding the wine and shallot, stirring any crystallized juices left clinging to the bottom (use a wooden spoon to prevent scratching). Let the wine reduce until almost dry, about 2 to 3 minutes, then add the stock and mushrooms. Bring to a boil and simmer until the liquid is syrupy, about 9 to 10 minutes.
Remove from heat, swirl in the butter (if using), and add the sage. Season to taste with salt and pepper. Spoon the sauce over the veal and serve.