The night before: Place the racks in the upper and lower thirds of your oven and preheat the oven to 350 degrees F. Arrange the bread slices in a single layer on two large baking sheets. Toast the bread in the oven until it is very lightly toasted and feels dry to the touch on top but is not yet turning brown, about 8 to 10 minutes, switching the pans’ positions on the upper and lower racks halfway through. Transfer to a cutting board and cut in half diagonally (this is easiest while the slices are still warm). Let cool slightly.
Meanwhile, butter a 9×13-inch casserole dish. In a small bowl, whisk together the granulated sugar and 1 teaspoon cinnamon. In a large bowl, whisk together the evaporated milk, milk, eggs, maple syrup, vanilla, lemon or orange zest, salt, and remaining 1/4 teaspoon cinnamon.
Fan the bread slices into the pan, arranging them however you like. Check out the photos above for inspiration, or make the arrangement your own.
Pour the custard mixture evenly over the top of the bread. Lightly press down the slices, sprinkle on the cinnamon-sugar mixture, and cover the pan with foil. Let sit for 15 minutes or refrigerate overnight.
When ready to bake: Place a rack in the center of the oven and preheat the oven to 375 degrees F. (If you’ve been refrigerating the French toast, remove it from the refrigerator now and let it come to room temperature.) Bake covered for 20 minutes, then uncover and continue baking until the bread is puffed and golden and no liquid seeps out of the toasts when lightly pressed, about 10 additional minutes. Remove from the oven and let rest a few minutes. Slice into squares and serve hot, with any and all of your favorite toppings.