Preheat oven to 350°F*. Line a baking sheet with parchment paper.
Slice the loaf of bread lengthwise into two halves. Place both halves of the bread onto the lined baking sheet, cut side up.
In a small bowl, combine the butter and garlic salt. Spread the butter mixture over the cut sides of the bread. Set aside while you make the sloppy joe filling.
Sloppy Joe
In a medium nonstick skillet over medium heat, cook the ground beef and onions until the beef is no longer pink (8-10 minutes). Drain grease. Stir in the garlic powder, mustard, ketchup, salt, pepper, and water, mixing until combined.
Assembly
Divide the sloppy joe mixture between both sides of the loaves and top with the mozzarella and cheddar cheese. Bake for 12-14 minutes or until golden brown.
Cut into pieces and serve with parsley for garnish.