Preheat your oven to 375°F (190°C) and lightly grease a baking dish.
Heat olive oil in a large skillet over medium heat until shimmering. Add sliced mushrooms and sauté until they release their moisture and turn golden brown, about 8 minutes, smelling earthy and hearing a gentle sizzle.
Add chopped sage to the mushrooms and cook for another minute until fragrant. Season with salt and pepper, then remove from heat and let cool slightly.
In a mixing bowl, combine shredded chicken, sautéed mushrooms with sage, heavy cream, and grated Parmesan. Stir everything together until well mixed and creamy, observing the mixture become slightly thickened and cohesive.
Pour the chicken and mushroom mixture into your prepared baking dish, spreading it out evenly with a spatula. This creates a layered base with a hearty, inviting appearance.
In a small bowl, toss breadcrumbs with melted butter until coated; this will give a golden, crispy topping. Sprinkle the breadcrumb mixture evenly over the casserole, ensuring full coverage to form a crunchy crust.
Place the dish in the oven and bake for about 25–30 minutes, or until the top is bubbling and the breadcrumbs are golden brown, filling your kitchen with a warm, toasted aroma.
Remove from oven and let sit for 5 minutes; the casserole will set slightly and offer a pleasing layered look with crispy topping and tender filling.
Slice into portions and serve hot, enjoying the creamy interior contrast with the crunchy breadcrumb topping, making every bite satisfying and beautifully layered.
