If possible, slice the challah the night before and let dry on a wire rack. Otherwise, dry the challah in a 300°F oven for about 15 minutes, then let cool while you prepare the egg mixture.
Preheat the oven to 250°F. In a large bowl, whisk together the eggs, milk, maple syrup, salt, cinnamon, cardamom, and nutmeg. Pour the custard into a 9-by-13-inch baking dish and add the bread in a single layer. Let soak for a few minutes, then turn the bread over and let soak for a few minutes longer.
Heat a No.9 Field Griddle over medium heat. Add a tablespoon of butter and enough slices of soaked bread to fill the griddle. Cook until browned on the bottom, 3 to 4 minutes, then flip the bread over and continue cooking until the bottom is browned, 3 to 4 minutes longer. Transfer to a baking sheet and keep warm in the oven while you cook the remaining bread, adding a tablespoon of butter with each batch.
When all of the French toast is cooked, dust the top with powdered sugar and serve right away with maple syrup.
Field Notes
