Grease a 9 by 13 inch baking dish and spread the cubed bread evenly inside.
Mash one banana in a large bowl. Whisk in the eggs, milk, cream, sugars, vanilla, cinnamon, nutmeg, and salt.
Pour the custard over the bread. Gently fold in the sliced bananas until evenly distributed.
Cover the dish tightly and refrigerate for at least 8 hours or overnight.
Preheat the oven to 350°F. Remove the casserole from the refrigerator while the oven heats.
Combine the flour, brown sugar, and cinnamon for the topping. Rub in the cold butter until coarse crumbs form.
Sprinkle the streusel evenly over the casserole and bake uncovered for 45 to 55 minutes, until the edges are golden and the center is set.
Cover loosely with foil if the topping browns too quickly. Let the casserole rest for 10 minutes before serving.
Serve warm with maple syrup, yogurt, fresh berries, or powdered sugar.