Preheat your oven to 375°F (190°C). Grease a baking dish to prepare for assembling the casserole.
Chop roasted vegetables into bite-sized pieces and shred the cooked chicken if not already prepared. Set aside.
Melt butter in a large skillet over medium heat, then whisk in flour to form a smooth roux. Cook for about a minute until lightly golden and fragrant.
Gradually whisk in milk, continuing to stir until the sauce thickens and becomes creamy, about 3-4 minutes. Stir in autumn spices, salt, and pepper to infuse warm flavors.
Add shredded chicken, roasted vegetables, and shredded cheese into the sauce, folding gently until combined and heated through.
Pour the mixture into your prepared baking dish, spreading it evenly.
Sprinkle bread crumbs and an additional handful of cheese over the top for a crispy, cheesy crust.
Bake in the preheated oven for 25-30 minutes, until bubbly and golden brown on top.
Remove from the oven and let rest for 5 minutes to settle before serving, revealing a bubbling, cheesy, and fragrant casserole.