Browning Mushrooms
Place the mushrooms in a single layer in a large hot skillet with the melted butter and hot oil. Cook on high heat for 1 minute and don’t stir, to caramelize the mushrooms. Then stir and continue cooking another 2-3 minutes, stirring occasionally, until the mushrooms are wilted.
Stir in the shallots and salt and cook another minute. Add the demi-glaceor stock and bring to a boil. Lower the heat and simmer for a couple of minutes.
Prepare the Veal
If you like, you can lightly dust the veal in 1/2 cup of flour placed in a pie pan for ease of dredging. This does promote additional browning and thickening of the sauce, but I prefer to cook the vealau natural, just sprinkle both sides of each scallop with salt and pepper.
Thicken the Sauce (optional)
The flour creates a fond on the bottom of the skillet and when you add the liquid back to the pan, scrape the bottom to release these flavorful tidbits and the remains of the flour will thicken the sauce slightly.
As an alternative to flour, use a cornstarch slurry, which leaves the sauce translucent, the flour makes an opaque sauce. Thoroughly combine 1 tablespoon cornstarch with 2 tablespoons cold water and drizzle into the hot sauce stirring constantly, until you reach a desired consistency; you won’t need to use the entire slurry, which would make the sauce too thick.
You can either leave the sauce in the pan and use another to sauté the veal, or pour the sauce into a bowl, use a spatula to scrape the pan clean. Wipe with a wet paper towel to clean out the pan so any residual sauce doesn’t burn while you cook the veal.
Cook in Batches
To avoid steaming the veal scallops, cook in batches, add a little more oil to pan as needed. Hold the cooked veal on a warm platter.
To Finish
Add the marsala wine and the sauce with mushrooms to the pan and bring to a boil; be sure to scrape the bottom of the pan to release any of the delicious tidbits from browning the veal.
If you want to use the cornstarch slurry, now is the time.
To finish, stir in the parsley and pour over the veal.
Time your side dishes to finish up by the time you add the sauce back to the skillet and everything is hot and ready together.