Preheat your oven to 350°F (175°C) and lightly grease your baking dish to prevent sticking.
Slice your peeled and cored apples into thin, even pieces and spread them evenly across the bottom of the prepared dish. This creates a sweet, tart layer that will soften and meld with the other ingredients.
Cut the cinnamon rolls into bite-sized pieces and scatter them over the apples, covering the entire surface for a gooey, layered effect.
In a mixing bowl, whisk together the eggs, milk, granulated sugar, vanilla extract, and cinnamon until well combined. This custard will soak into the rolls and apples as it bakes.
Pour the custard evenly over the entire casserole, ensuring all the ingredients are coated and some liquid seeps between the layers. This will create a soft, cohesive bake.
Sprinkle the brown sugar on top for a caramelized, sweet crust as it bakes, and add a pinch of cinnamon if desired for extra aroma.
Place the dish in the oven and bake uncovered for about 35 minutes, or until the custard is set and the top is golden brown and bubbling.
Once baked, remove the casserole from the oven and let it rest for a few minutes. The edges will be slightly crisp, and the center will be soft and custardy.
Serve warm, optionally topped with whipped cream or a drizzle of caramel sauce for extra indulgence.
