Preheat your oven to 375°F (190°C). Grease a 9x13-inch baking dish to prevent sticking. Layer the sliced apples evenly on the bottom of the dish and toss lightly with granulated sugar and cinnamon to coat. Spread the apple slices out in a uniform layer so they’ll bake evenly.
Cut the cinnamon rolls into small, bite-sized pieces and scatter them over the apple layer. This creates a layered effect with soft dough nestled among the fruit, ready to absorb the surrounding flavors as it bakes.
In a mixing bowl, whisk together the eggs, milk, vanilla extract, and melted butter until the mixture is smooth and slightly frothy. This custard mixture will pour evenly over the layers, infusing them with richness and helping everything bind together.
Pour the custard evenly over the casserole, making sure it seeps into every crevice and around the cinnamon roll pieces. You might see the liquid pooling in some spots—that’s perfect, as it will create a moist, tender bake.
Place the baking dish in the preheated oven and bake for about 35 minutes, or until the top is golden brown and bubbling around the edges. The cinnamon roll pieces should be puffed and slightly crisp on top, with the apples soft and fragrant.
Remove the casserole from the oven and let it rest for a few minutes. Drizzle with any reserved icing from the cinnamon rolls for an extra touch of sweetness and gooey texture. Serve warm, with the layers visibly combined with a delightful mix of soft and crunchy textures.
