Preheat your oven to 375°F (190°C). Grease a baking dish with butter to prepare for layering.
Peel, core, and dice the apples into roughly 1/2-inch pieces. Toss them in a small bowl with a sprinkle of cinnamon and a pinch of salt. Spread the apple chunks evenly on a baking sheet and bake for about 15 minutes until slightly softened and fragrant.
While the apples roast, tear or cube the stale bread into bite-sized pieces. Place them in a large mixing bowl.
In a small bowl, whisk together the eggs, milk, melted butter, chopped sage, ground cinnamon, salt, and black pepper until well combined. Pour this mixture over the bread and gently fold to coat all the pieces, allowing them to soak up the custard for about 5 minutes.
Layer half of the soaked bread in the prepared baking dish. Spread the roasted apple chunks evenly over the bread layer.
Add the remaining soaked bread on top, pressing down gently to secure everything in place.
Place the assembled casserole in the oven and bake for 30-40 minutes, until the top is golden brown, bubbling around the edges, and a toothpick inserted in the center comes out clean.
Remove from the oven and let it rest for 5 minutes before serving. The top should be crispy and the inside soft, with warm, fragrant apple and sage filling.