Preheat your oven to 350°F and line a large baking sheet with foil or parchment paper. While the oven heats, cut your French bread loaf in half lengthwise to create two long, flat surfaces. Mix the softened butter with garlic salt in a small bowl until well combined, then generously spread this mixture across the cut sides of both bread halves. I like to use an offset spatula to get an even, thin layer—it helps the bread toast evenly without becoming too greasy. Place the prepared bread halves cut-side up on your prepared baking sheet and set aside.

Heat a large skillet over medium-high heat and add the ground beef along with the diced onion. Cook for 8-10 minutes, breaking up the meat with a spoon as it cooks until it's fully browned and the onions are softened. Carefully pour off any excess fat from the pan, leaving just a light coating. Add the garlic powder, yellow mustard, ketchup, salt, pepper, Worcestershire sauce, brown sugar, and water to the cooked meat. Stir everything together and simmer for 2-3 minutes to allow the flavors to meld—I find that letting these flavors sit for just a minute or two makes a noticeable difference in depth. The sauce should be thick but still slightly moist, not dry or watery.

Divide the warm sloppy joe mixture from Step 2 evenly between the two garlic butter bread halves, spreading it in an even layer. Top each half with the shredded mozzarella and cheddar cheeses, distributing them evenly so you get cheese in every bite. Place the assembled bread on the baking sheet in the preheated 350°F oven and bake for 12-14 minutes, until the cheese is completely melted and bubbly and the edges of the bread are golden brown. The cheese should be slightly browned in spots for the best flavor.

Remove the baked sloppy joe bread from the oven and let it cool for just 1-2 minutes so the cheese sets slightly and won't slide off when you slice. Using a sharp serrated knife, cut each bread half into individual portions—typically 4-6 pieces per half depending on your preference. Transfer to a serving platter and sprinkle the fresh chopped parsley over the top for a pop of color and freshness. Serve immediately while the bread is still warm and the cheese is at its melted best.