Preheat your oven to 425°F (220°C). Using a serrated knife, slice the baguette in half lengthwise so you have two long halves. If you prefer smaller servings, cut into 4–6 diagonal pieces after slicing. Place the baguette halves cut-side up on a baking sheet lined with parchment or foil.

Make garlic butter

In a small bowl, combine the softened unsalted butter, minced garlic, chopped parsley, salt, and black pepper. Mix thoroughly until smooth and evenly distributed.

Spread butter and add cheeses

Generously spread the garlic butter mixture over the cut surfaces of the baguette halves. Evenly sprinkle the grated Parmesan over both halves, then follow with the shredded mozzarella to create a cheesy layer.

Optional Toppings (for chicken Alfredo variation)

Add chicken and Alfredo (optional)

If making the Chicken Alfredo-style baguette au gratin, evenly distribute the cooked sliced or shredded chicken over the cheese layer, then spoon the prepared Alfredo sauce in thin ribbons over the chicken and cheese. Avoid drenching the bread so it remains crisp under the cheese.

Finishing (for broiling)

Bake and brown

Place the assembled baguette in the preheated oven and bake for 8–10 minutes, or until the cheese has melted and the edges of the bread begin to turn golden. For a browned, gratin-style top, switch the oven to broil on high for 1–2 minutes—watch closely to prevent burning—until the mozzarella and Parmesan are bubbly and lightly browned.

Finish and serve

Remove from oven and let rest 1–2 minutes. If desired, drizzle a small amount of olive oil over the top and sprinkle additional chopped parsley. Cut into serving pieces (if not already) and serve hot. Add a pinch of chili flakes to individual portions to taste.