Heat the butter or oil in a large skillet over medium-high heat. Add the sliced veal or chicken and cook until browned and cooked through, about 5-7 minutes. Remove the meat from the skillet and set aside.
In the same skillet, add the chopped onion and cook until soft and translucent, about 3-4 minutes. Add the minced garlic and cook for another 1 minute.
Pour in the white wine or chicken broth, scraping up any browned bits from the bottom of the pan. Allow the liquid to simmer and reduce slightly, about 2-3 minutes.
Stir in the heavy cream and Dijon mustard. If you prefer a thicker sauce, mix the flour with a small amount of water to make a slurry and stir it into the sauce. Simmer for 3-5 minutes, or until the sauce has thickened to your liking.
Add the cooked meat back to the skillet, stirring to coat it in the sauce. Cook for an additional 2-3 minutes, or until the meat is heated through and well combined with the sauce.
Season with salt and pepper to taste. Garnish with chopped fresh parsley.
Serve the Geschnetzeltes over rice, noodles, or alongside your favorite side dishes.
Enjoy your meal!