Preheat the oven to 350˚F. Melt the butter in a saucepan over medium-low heat. Add the onions and saute for 8-10 minutes. Set aside.

Put the seasoned breadcrumbs, freshly grated parmesan, eggs, milk, black pepper, and salt in a large bowl. Stir to combine.

Add the ground beef, ground pork sausage, and onions. Mix well.

Form the mixture into a loaf shape on a foil-lined, rimmed baking sheet.

In a small bowl, mix together the ketchup, brown sugar, ground mustard, and Worcestershire sauce. Spread half of the mixture over the meatloaf.

Bake at 350˚F for 40 minutes. Pour the remaining sauce over the top and continue to bake for an additional 20-25 minutes until it reaches an internal temperature of 160˚F. Let stand for 10 minutes before slicing. Garnish with chopped parsley.