Grease 9×13-inch baking dish with butter or nonstick cooking spray. Add bread cubes in even layer.

In large mixing bowl, whisk together 8 large eggs, 1¼ cups heavy cream, ¾ cup whole milk, ¾ cup brown sugar, 2 tablespoons maple syrup, 1 tablespoon vanilla, 1½ teaspoons cinnamon, and pinch of salt until smooth and fully combined.

Pour custard mixture evenly over bread cubes. Press bread down gently with spatula so every cube is coated. If using fruit, scatter 1 to 2 cups blueberries or other fruit over top.

Cover tightly with plastic wrap and refrigerate at least 6 hours, or overnight is recommended.

In small bowl, combine ½ cup all-purpose flour, ½ cup brown sugar, ¾ teaspoon cinnamon, and pinch of salt. Add ½ cup cold cubed butter and cut in with pastry cutter or two forks until mixture resembles coarse, pea-sized crumbles. Stir in ½ cup chopped pecans or walnuts if using.

Cover and refrigerate streusel until ready to bake. Do not add to casserole overnight.

Remove casserole from refrigerator 30 minutes before baking. Preheat oven to 350°F.

Sprinkle cold streusel evenly over top of casserole. Bake uncovered 45 to 55 minutes, until center is set and streusel is deep golden brown. If top browns too quickly, tent loosely with foil for remaining bake time.

Check doneness by inserting knife into center — should come out clean with no wet custard.

Rest casserole 10 minutes before serving. Serve warm with powdered sugar, maple syrup, whipped cream, or fresh fruit.