Step 1: Preheat your oven to 350°F (175°C) and grease a 9×13 inch baking dish with butter or non-stick spray.
Step 2: Peel and slice the peaches into 1/2-inch thick wedges. Place the peach slices in the bottom of the prepared baking dish in an even layer.
Step 3: Cut the Challah or Brioche bread into 1-inch cubes and place them on top of the peaches, spreading them out to cover the fruit.
Step 4: In a large mixing bowl, whisk together the eggs, whole milk, heavy cream, brown sugar, vanilla extract, cinnamon, nutmeg, and salt until well combined and the sugar has dissolved.
Step 5: Slowly pour the egg mixture over the bread cubes, ensuring every piece of bread is moistened. Use a spatula to gently press down on the bread to help it absorb the liquid.
Step 6: If time allows, let the dish sit for 20 minutes (or refrigerate overnight) to allow the custard to fully soak into the bread.
Step 7: Bake for 45-50 minutes, or until the top is golden brown and the custard is set in the center.
Step 8: Remove from the oven and let rest for 5-10 minutes before serving. Dust with powdered sugar and serve with maple syrup if desired.