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.
Enjoy your meal!