Preheat the oven to 350°F (175°C) and grease your baking dish with butter for easy removal.
Arrange the bread slices evenly in the prepared baking dish, covering the bottom in a single layer.
Spread the sliced apples across the bread, creating a colorful layer that will soften during baking.
In a mixing bowl, whisk together the eggs, milk, brown sugar, cinnamon, and vanilla until smooth and slightly frothy.
Pour the custard mixture evenly over the bread and apples, ensuring all slices are soaked and the apples are coated with the custard.
Press down gently on the bread to help it absorb the custard and ensure even soaking.
Cover the dish with plastic wrap and refrigerate overnight, or at least 4 hours, to allow the bread to soak thoroughly.
Remove the dish from the refrigerator and let it sit at room temperature for 15 minutes before baking to promote even cooking.
Bake the casserole uncovered in the preheated oven for 35-45 minutes, until the top is golden brown and the custard is set around the edges with a slight jiggle in the center.
Once baked, remove from the oven and let it rest for a few minutes to settle. The smell will be warm and inviting, and the surface will be beautifully golden with slightly crisp edges.
Serve warm, optionally drizzled with maple syrup or topped with whipped cream, and enjoy the rich, fluffy, cinnamon-spiced layers with tender apples.