Start by cutting your French bread (or brioche/challah) into 1-inch cubes. For the best texture, use day-old bread—its firmness will absorb the custard without becoming soggy. If your bread is fresh, just leave it out for a few hours to dry out slightly.

In a large mixing bowl, whisk together the eggs, milk, heavy cream, both sugars, vanilla extract, cinnamon, nutmeg (if using), and a pinch of salt. The custard should be smooth and well-blended.

Place the bread cubes into a greased 9x13-inch baking dish, evenly distributing them across the bottom. Slowly pour the custard mixture over the bread, making sure all the cubes are coated. Gently press the bread down to help it soak up the custard. Let it sit for about 10-15 minutes for the bread to absorb the liquid—if you have more time, you can cover it with plastic wrap and refrigerate overnight.

In a separate bowl, combine the flour, brown sugar, cinnamon, nutmeg, and salt for the topping. Add the cold, cubed butter and use a pastry cutter or your hands to work the butter into the dry ingredients until the mixture becomes crumbly. Sprinkle the topping evenly over the casserole.

Preheat your oven to 350°F (175°C) and bake the casserole for 45-50 minutes or until it’s golden brown and slightly puffed up. If the top gets too brown before the center is fully cooked, cover it with aluminum foil and continue baking until done.

Once baked, remove the casserole from the oven and let it cool slightly. Serve it warm, topped with fresh berries, a dusting of powdered sugar, or a drizzle of maple syrup.