Preheat the oven to 400°F. Slice the bread in half lengthwise and place on a lined baking sheet.
Mix butter, garlic powder, and salt. Spread evenly on bread slices. Bake for 10 minutes until golden brown. Set aside. Reduce oven temperature to 350°F.
In a skillet over medium heat, brown the ground beef with onions. Drain excess grease.
Add water, ketchup, mustard, garlic powder, Worcestershire sauce, salt, and pepper to the beef. Simmer for 5 minutes.
Add the cheddar cheese and pepper jack cheese to a bowl and mix until combined well.
Spread the Sloppy Joe mixture evenly on the bread halves. Top with both cheeses.
Bake at 350°F for 10-12 minutes until the cheese is melted. Optional: Broil for 1 minute for a golden top.
Remove from oven, let rest for a few minutes. Garnish with parsley, slice, and serve.
