Pre-heat oven to 325° and arrange rye bread slices on a baking sheet.Brown the ground beef and sausage together and drain the excess fat. Stir in the oregano, worcheshire sauce, garlic powder, salt, and pepper into cooked meat and set aside.Create a roux by melting the butter over medium heat. Whisk in the flour and continue cooking for a few minutes to get the flour taste to cook out. Slowly whisk in warmed milk. Whisk in mustard powder and white pepper. Remove from heat.Add shredded cheese ½ cup at a time, stirring to melt in between additions. Stir in meat mixture.Top rye bread slices with meat mixture. Bake for 15 minutes. Top with chopped parsley, if desired, and serve immediately.