Place the cubed ciabatta into a buttered 9X13” casserole baking dish and set aside.
In a large bowl, whisk together the eggs, milk, heavy cream, vanilla extract, cinnamon, brown sugar, nutmeg, and salt until well combined.
Pour the egg mixture over the ciabatta cubes and gently toss the bread until all of the pieces are coated. Cover the casserole dish with foil and put in the refrigerator. Let the bread soak up the wet ingredients for (at least) one hour to overnight.
When ready to bake, preheat oven to 350 degrees and remove the casserole dish from the refrigerator.
In a small bowl, combine the flour, brown sugar, cinnamon, salt, and breadcrumbs and stir until combined. Dump the butter into the bowl and use a pastry cutter or fork to cut the butter into the ingredients. You want the topping to be chunky and resemble pebbles.
Remove the foil from the dish and sprinkle on the topping. Place the foil back onto the dish and bake in the oven for 25 minutes.
After 25 minutes, remove the foil and continue to bake. If you prefer a more custard-y (bread pudding type) French toast casserole, bake for another 20 minutes. If you prefer a crispier, more firm French toast casserole, bake for another 35-40 minutes.
Optional: dust the casserole with confectioner’s sugar and serve with butter and maple syrup. Enjoy!