Preheat your oven to 375°F (190°C) and have a baking dish ready.
In a large skillet, heat the olive oil over medium heat. Add the diced onion and cook, stirring occasionally, until it turns soft and begins to caramelize, about 8 minutes. The onions should smell sweet and look golden brown.
Add the dried sage to the onions and cook for another 30 seconds to release its aroma.
In a mixing bowl, combine the shredded chicken, roasted vegetables, and the sautéed onion mixture. Mix gently until well combined.
Transfer the chicken and vegetable mixture into the prepared baking dish, spreading it out evenly.
In a small bowl, mix the breadcrumbs with melted butter and a pinch of salt and pepper to taste. Stir until the crumbs are coated and slightly clumped together.
Sprinkle the buttery breadcrumbs evenly over the top of the chicken and vegetable mixture. This will create a golden, crispy topping.
Place the dish in the oven and bake for about 30-35 minutes, or until the topping is golden and bubbling around the edges.
Remove the casserole from the oven and let it sit for 5 minutes to settle before serving. The aroma of sage and caramelized onion should fill the kitchen.
Scoop portions onto plates and enjoy this warm, hearty fall dish with a side salad or crusty bread.
