Set your oven to preheat at 350°F (180°C), and prepare a large baking tray by lining it with parchment paper or foil for easy cleanup.
Slice the ciabatta bread horizontally, creating two long halves. Spread a generous layer of the softened butter on the cut sides, then evenly dust with garlic powder. Season lightly with salt and pepper. Set the prepared bread aside.
Warm a skillet or Dutch oven over medium-high heat. Pour in the olive oil and add the ground beef. Break up the meat as it cooks, continuing until there is no visible pink. Drain any excess fat if necessary.
Introduce the finely chopped onion into the pan, and sauté until it becomes soft and translucent. Stir in the minced garlic and let it cook briefly, around 30 seconds.
Pour in the ketchup followed by water, then add the brown sugar, mustard, Worcestershire sauce, and a pinch each of salt and pepper. Stir thoroughly to combine all the flavors.
Lower the heat and let the mixture simmer, partially covered, for 20 to 30 minutes. Stir occasionally until the sauce thickens to your desired consistency. Taste and adjust the seasoning if needed.
Spoon the hot sloppy joe filling evenly across both halves of the buttered ciabatta. Sprinkle mozzarella and cheddar cheeses over the top of the meat.
Place the assembled bread into the preheated oven and bake for about 10 minutes, or until the cheese is melted and the bread edges are lightly browned and crisp.
Once baked, remove from the oven, slice into portions, and optionally garnish with freshly chopped parsley. Serve immediately.