Preheat your oven to 350°F. Line a baking sheet with parchment paper. Slice the loaf of Italian bread lengthwise right down the middle and place both halves onto the baking sheet, cut side up.
In a small mixing bowl, stir together the softened unsalted butter and garlic salt until smooth. Spread this butter evenly over both bread halves, going all the way to the edges.
Place a medium skillet on the stove over medium heat. Add the ground beef and chopped yellow onion. Break the meat apart and cook for 8-10 minutes until completely browned with no pink remaining. Carefully drain the excess grease.
Sprinkle the garlic powder, kosher salt, and black pepper over the beef. Pour in the ketchup, yellow mustard, and water. Stir until the sauce is smooth and thick, then let it simmer gently for one minute before removing from heat.
Spoon the warm sloppy joe filling onto the garlic bread, dividing it evenly between both halves. Spread it out to the edges and press the meat down slightly so it stays in place.
Sprinkle the shredded mozzarella and sharp cheddar cheese evenly over the meat.
Bake in the preheated oven for 12-14 minutes, or until the cheese is completely melted and bubbly and the bread edges are golden brown.
Remove from the oven, let cool for a minute, slice into portions, garnish with chopped parsley, and serve hot.