Melt butter in a large frypan over medium-high heat. Add beef and pork mince and cook until no longer pink. Gradually mix in flour while stirring continuously until the mixture is thick and pasty. You may not need all of the flour. Slowly whisk in the milk using a fork so that lumps do not form. Stir in stock powder. Continue cooking and stirring constantly until the mixture reaches your desired consistency. The longer it cooks, the thicker it will get.
Toast slices of white bread and top with the meat. Garnish with a little fresh parsley and serve.