Step 1: Prepare the Potatoes and Aromatics
Preheat your oven to 375°F (190°C). Generously grease a 9×13 inch (23x33cm) baking dish with butter or non-stick spray. In a very large bowl, combine the thinly sliced potatoes, sliced onion, and minced garlic. Drizzle with olive oil and sprinkle with smoked paprika, garlic powder, onion powder, dried thyme, salt, and black pepper. Use your hands to toss everything thoroughly, ensuring every slice is lightly coated with oil and seasonings. This step is crucial for flavoring the potatoes throughout.

Step 2: Create the Luxurious Cream Sauce
In a medium saucepan over medium heat, whisk together the heavy cream and chicken broth. In a small separate bowl, whisk the flour into about ¼ cup of the cold cream mixture until it forms a smooth slurry with no lumps. Gradually whisk this slurry back into the warm cream in the saucepan. Add the dry mustard and nutmeg. Cook, whisking constantly, for 5-7 minutes until the sauce thickens noticeably and can coat the back of a spoon. It should be the consistency of a thin gravy. Season with salt and white pepper. Remove from heat.

Step 3: Assemble the First Potato Layer and Sauce
Arrange half of the seasoned potato and onion mixture in the bottom of the prepared baking dish, spreading it into an even layer. Pour half of the prepared cream sauce evenly over this first potato layer, shaking the dish gently to help it seep down.

Step 4: Create the Flavor-Packed BBQ Chicken Layer
In the now-empty large bowl, combine the shredded chicken, 1 cup of the BBQ sauce, apple cider vinegar, and liquid smoke (if using). Stir until the chicken is evenly and thoroughly coated. Spoon this BBQ chicken mixture evenly over the first sauced potato layer in the baking dish, spreading it to the edges.

Step 5: Complete the Layering with Final Potatoes and Sauce
Carefully arrange the remaining half of the potato and onion mixture over the chicken layer, creating the top stratum. Pour the remaining cream sauce evenly over the top, again shaking the dish gently to distribute. Cover the baking dish tightly with aluminum foil.

Step 6: Bake Until Potatoes are Tender
Place the covered dish in the preheated oven. Bake for 50 minutes. After this time, remove the dish from the oven and carefully remove the foil. The potatoes should be just tender when pierced with a knife. Increase the oven temperature to 400°F (200°C).

Step 7: Add Cheese, BBQ Glaze, and Finish Baking
In a bowl, combine the shredded cheddar, Monterey Jack, and Parmesan cheeses. Sprinkle this cheese blend evenly over the entire surface of the casserole. Drizzle the remaining ½ cup of BBQ sauce in artistic lines or dollops over the cheese. Return the uncovered dish to the 400°F oven. Bake for an additional 20-25 minutes, or until the cheese is completely melted, bubbly, and beginning to brown in spots, and the top is gloriously golden.

Step 8: Garnish, Rest, and Serve
Remove the bake from the oven. Immediately sprinkle with crumbled bacon (if using) and sliced green onions. Let the casserole rest for 15 minutes before serving. This critical resting period allows the creamy sauce to thicken and set, ensuring you can cut clean, beautiful squares that hold their shape. Garnish with fresh herbs just before serving