Preheat the oven to 350°F (175°C) and arrange the bread slices on two large baking sheets in a single layer. Toast for 8-10 minutes, switching the pans halfway through. The bread should be dry but not browned. After toasting, cut the slices diagonally and let them cool slightly.
While the bread cools, butter a 9x13-inch casserole dish. In a small bowl, mix together 3 tablespoons of sugar and 1 teaspoon of cinnamon. In a large bowl, whisk together the evaporated milk, milk, eggs, maple syrup, vanilla extract, zest, salt, and remaining 1/4 teaspoon cinnamon.
Arrange the bread slices in the casserole dish in a fan shape or any pattern you like. Pour the custard mixture evenly over the bread, pressing the slices gently. Sprinkle the cinnamon-sugar mixture over the top. Cover the dish with foil and refrigerate overnight or for at least 15 minutes.
Preheat the oven to 375°F (190°C). Remove the casserole from the refrigerator and let it come to room temperature. Bake the covered casserole for 20 minutes, then uncover and bake for another 10 minutes or until the bread is golden and puffed. Let it rest for a few minutes before slicing.
Slice into squares and serve with your favorite toppings like maple syrup, fresh berries, powdered sugar, or a dollop of Greek yogurt.