Preheat the oven to 350°F (175°C). Line a baking sheet with parchment paper for easy cleanup.
Slice the Italian bread lengthwise into two halves and place them on the prepared baking sheet, cut side up.
Combine the softened butter and garlic salt in a small bowl. Evenly spread the mixture over the cut sides of the bread. Set aside.
In a medium skillet over medium heat, brown the ground beef with the chopped onion until the meat is fully cooked and no longer pink, about 8-10 minutes. Drain any excess grease.
Stir the garlic powder, mustard, ketchup, kosher salt, black pepper, and water into the cooked beef. Mix thoroughly to combine and allow the mixture to simmer for 2-3 minutes.
Spoon the sloppy joe mixture evenly over the buttered bread halves. Top with shredded mozzarella and sharp cheddar cheese.
Bake in the preheated oven for 12-14 minutes, or until the cheese is melted and golden brown.
Slice into individual portions, garnish with fresh parsley, and serve immediately.