Sautee beef strips and onion in butter until done. Season with salt and pepper, set aside and keep warm in a low heat oven or microwave at low power. Prepare the sauce combining melted butter with the flour to make a roux. Add the dry mustard and heat at medium, adding the beef stock. Add the sour cream, whisking to fully incorporate into the mixture. Spoon the meat over potatoes or cooked pasta noodles. Pour sauce over the meat and garnish with fresh chopped parsley.