Step 1: Roast the Garlic and Cook the Pasta
Preheat oven to 400°F (200°C). Slice the top ¼ off the head of garlic to expose the cloves. Drizzle with olive oil, wrap in foil, and roast for 40 minutes until soft and golden. Let cool, then squeeze the soft, fragrant roasted garlic paste into a small bowl. Reduce oven to 375°F (190°C) for the casserole. Cook pasta in salted boiling water for 2-3 minutes LESS than package directions for al dente. Drain and set aside.

Step 2: Prepare the BBQ Chicken
In a medium bowl, combine the shredded chicken with ¾ cup of the BBQ sauce and the smoked paprika. Toss until evenly coated. Set aside to let the flavors marry.

Step 3: Create the Luxurious Creamy Garlic Sauce
In a large Dutch oven or heavy pot, melt 4 tbsp butter over medium heat. Whisk in the flour and cook, stirring constantly, for 2 minutes to make a blonde roux. Gradually whisk in the warm milk and chicken broth until smooth. Cook, whisking frequently, until the sauce thickens and coats the back of a spoon, about 8-10 minutes. Reduce heat to low. Whisk in the roasted garlic paste until fully incorporated. Add the cubed cream cheese and sour cream, whisking until completely smooth and luxurious. Stir in onion powder, dry mustard, white pepper, and salt to taste. Remove from heat.

Step 4: Combine Pasta, Chicken, and Sauces
To the pot with the creamy garlic sauce, add the drained al dente pasta and the BBQ chicken mixture. Gently fold everything together until the pasta and chicken are evenly coated. Now, take the remaining ½ cup of BBQ sauce and drizzle it over the mixture. Using a spatula, gently fold just 2-3 times to create a beautiful, marbled effect—do not fully combine.

Step 5: Incorporate the First Cheese Blend
In a large bowl, combine the shredded white cheddar, Gruyère (or Gouda), 1 cup of the mozzarella, and ½ cup of the Parmesan cheese. Reserve the remaining mozzarella and Parmesan for the topping. Gently fold this cheese blend into the pasta mixture until just distributed and melted into the warmth of the sauce.

Step 6: Assemble and Add the Crispy Topping
Pour the entire cheesy pasta mixture into a greased 9×13 inch (23x33cm) baking dish, spreading it evenly. In a small bowl, mix the Panko breadcrumbs, melted butter, chopped parsley, and 1 tsp garlic powder. Sprinkle the remaining ½ cup mozzarella and ½ cup Parmesan cheese over the pasta. Then, evenly sprinkle the buttery Panko mixture over the cheese.

Step 7: Bake to Golden, Bubbling Perfection
Place the baking dish on a rimmed baking sheet (to catch any bubble-overs). Bake, uncovered, at 375°F for 25-30 minutes, or until the topping is deep golden brown, the sauce is bubbling vigorously around the edges, and the cheese is fully melted. If the top browns too quickly, tent loosely with foil.

Step 8: Rest, Garnish, and Serve
Remove from the oven and let the casserole rest for 10-15 minutes. This allows the sauce to thicken and set for perfect serving. Garnish liberally with sliced green onions and fresh parsley or chives just before bringing this magnificent, aromatic bake to the table.