Grease a 9 x 13 casserole dish. Distribute the torn bread evenly in the dish. In a medium mixing bowl, whisk together the eggs, milk, cream, brown sugar, vanilla, and bourbon (if using) until smooth and sugar is dissolved. Pour the custard evenly over the bread, pressing the bread down into the mixture gently until fully coated. Cover with plastic wrap and refrigerate overnight.
To make the topping: in a mixing bowl, stir together the flour, brown sugar, cinnamon, salt, and nutmeg. Using a pastry blender, cut in the cold butter until the mixture is crumbly. Cover an refrigerate overnight.
Preheat oven to 350 degrees. Uncover the French toast and sprinkle it evenly with all of the cinnamon crumble mixture. Bake for 45 minutes, until puffed up, browned and set in the center. Slice into squares and serve while warm with pure maple syrup.