Preheat the oven to 350 degrees F (175 degrees C). Grease rimmed baking sheets lightly.

Heat bacon fat in a pan over medium heat. Saute onion and garlic in the hot pan until soft, 5 to 7 minutes. Remove from heat and allow to cool.

While mixture cools, cut the loaf of bread in half horizontally. Scoop out the white bread, leaving a boat-like shell of crust; save crust for pizza bread or garlic toast. Tear white bread into pieces (should yield 5 to 6 cups) and place into a bowl.

Combine milk, chicken broth, and eggs in a bowl. Pour over the torn bread. Mash bread into small pieces with your fingers. Stir in cooled onion and garlic; mix well. Add parsley, salt, Worcestershire, mustard, Italian seasoning, and pepper. Mix very well.

Combine ground beef and sausage in a large mixing bowl using your hands. Add bread mixture, about 1 cup at a time, mixing well with your hands between each addition, until all is combined evenly.

Form meatballs and place on the prepared baking sheets.

Bake until meatballs are browned and no longer pink in the centers, about 30 minutes.