Preheat the oven to 375°F (190°C). Grease a 9×13-inch baking dish lightly with cooking spray or olive oil.

In a small bowl, mix melted butter with minced garlic until well combined. Brush each slice of bread generously with the garlic butter mixture.

Arrange the bread slices on a baking sheet and toast in the preheated oven for 5–7 minutes, or until golden brown and slightly crisp.

While the bread is toasting, heat marinara sauce with water in a large skillet over medium-low heat. Add the meatballs and simmer for 10 minutes if frozen, or 5 minutes if fresh, stirring occasionally to coat evenly.

Place the toasted bread slices evenly across the bottom of the prepared baking dish. Spoon the meatballs and sauce evenly over the bread layer.

Sprinkle half of the shredded mozzarella cheese over the meatballs, followed by a few slices of provolone cheese. Top with grated Parmesan cheese and Italian seasoning.

Bake uncovered in the preheated oven for 20–25 minutes, or until bubbly and the cheese is melted and golden.

Remove from the oven and let cool slightly. Garnish with freshly chopped parsley if desired, then slice and serve warm.