Step 1: Prepare and Season the Potatoes and Chicken
Preheat your oven to 400°F (200°C). Grease a 9×13 inch (23x33cm) baking dish. In a large bowl, toss the cubed potatoes and chicken pieces with olive oil, garlic powder, onion powder, smoked paprika, dried thyme, salt, and pepper until evenly coated. Spread the mixture in a single layer on a large, rimmed baking sheet.

Step 2: Roast for Depth of Flavor
Roast the potato and chicken mixture on the baking sheet for 25-30 minutes, stirring halfway, until the potatoes are tender and starting to brown at the edges, and the chicken is cooked through. This step concentrates flavor and ensures everything is perfectly cooked before the final bake. Remove from oven and reduce oven temperature to 375°F (190°C).

Step 3: Create the Velvety Cheese Sauce
While the potatoes and chicken roast, make the sauce. In a large saucepan, melt the butter over medium heat. Whisk in the flour and cook for 1-2 minutes to form a blonde roux. Gradually whisk in the warm milk and chicken broth until smooth. Cook, whisking frequently, until the sauce thickens enough to coat the back of a spoon, about 8-10 minutes. Remove from heat.

Step 4: Finish the Signature Sauce
Take the saucepan off the heat. Whisk in 1 ½ cups of the shredded cheddar cheese and all of the Monterey Jack cheese until fully melted and smooth. Then, whisk in the sour cream, Dijon mustard, and optional cayenne pepper until the sauce is creamy and homogeneous.

Step 5: Combine and Assemble the Bake
Transfer the roasted potatoes and chicken to the prepared 9×13 inch baking dish. Sprinkle the diced red onion and half of the crumbled bacon over the top. Pour the creamy cheese sauce over everything, gently stirring to ensure even coating. Spread the mixture into an even layer.

Step 6: Apply the Loaded Topping
Sprinkle the remaining 1 cup of shredded cheddar cheese evenly over the top, followed by the remaining crumbled bacon.

Step 7: Bake to Bubbling, Golden Perfection
Bake, uncovered, at 375°F (190°C) for 20-25 minutes, or until the cheese is completely melted, the sauce is bubbling vigorously around the edges, and the top is golden brown.

Step 8: Garnish and Serve
Remove the casserole from the oven and let it rest for 10-15 minutes. This allows the sauce to thicken slightly for perfect serving. Garnish with the remaining sliced green onions and fresh chives or parsley. Serve hot, with a dollop of cool sour cream on top of each portion.