Preheat your oven to 400°F (200°C) and line a baking sheet with parchment or foil for easy cleanup.

In a small bowl, stir together 5 tablespoons of melted butter, 1 tablespoon of garlic powder, and the chopped parsley until combined. Arrange the halved French bread on the prepared baking sheet with the cut sides facing up. Generously brush the garlic butter mixture over each half, covering every edge.

Bake the bread for 4 to 5 minutes, just until the edges begin to turn golden and crisp. Remove from the oven and set aside, keeping the oven on.

While the bread toasts, pour the Alfredo sauce into a medium saucepan set over medium heat. Add the remaining 3 tablespoons of butter, 1/4 cup of the Parmesan cheese, the remaining 1/2 teaspoon of garlic powder, onion powder, smoked paprika, salt, and black pepper. Stir frequently until the butter melts and the sauce is smooth and warmed through, about 5 minutes. Remove from heat.

Spread half of the warm Alfredo sauce evenly over the toasted bread halves, letting it soak slightly into the crispy surface. Distribute the chopped chicken evenly over the sauce, then sprinkle the remaining 1/4 cup Parmesan along with the Gruyère and Monterey Jack cheeses on top.

Return the bread to the oven and bake for 8 to 10 minutes, or until the cheese is melted, bubbling, and beginning to brown in spots.

Slice the bread into generous 2-inch pieces, garnish with more fresh parsley, and serve immediately with the remaining Alfredo sauce on the side for dipping or drizzling.