Prepare the caramel base: In a small saucepan over medium heat, melt the butter, then add brown sugar and maple syrup. Stir until sugar dissolves completely (about 3-4 minutes). Pour this mixture evenly across the bottom of a greased 9×13 baking dish.
Arrange the bread: Place the bread slices in a single layer over the caramel mixture, slightly overlapping if necessary to fit all pieces.
Make the custard mixture: In a large bowl, whisk together eggs, half-and-half, vanilla extract, cinnamon, nutmeg, and salt until completely combined and slightly frothy.
Pour custard over bread: Slowly and evenly pour the egg mixture over the bread slices, making sure all pieces get soaked. Press down gently on the bread to help absorption.
Cover and refrigerate: Tightly cover the baking dish with plastic wrap and refrigerate for at least 8 hours or overnight (up to 24 hours). This resting time is crucial for the bread to fully absorb the custard.
Preheat before baking: When ready to bake, remove the dish from the refrigerator and let it sit at room temperature while you preheat the oven to 350°F (175°C).
Bake to perfection: Uncover the dish and bake for 40-45 minutes until the top is golden brown and the center is set (not jiggly). If the top browns too quickly, loosely cover with foil for the remaining baking time.
Rest before serving: Allow the French toast to rest for 5-10 minutes before serving. This helps the caramel set slightly and makes serving easier.
Serve with style: Serve warm, spooning some of the caramel sauce from the bottom of the dish over each portion. Garnish with fresh berries and a dusting of powdered sugar if desired.