To make casserole base: Preheat oven to 300 F. Cut bagels into 1-inch cubes. Spread on baking sheet and toast 10 minutes until slightly dry.
Lightly grease 9-by-13-inch baking dish. Add toasted bagel cubes and half the berries.
Whisk eggs, milk, cream, maple syrup, brown sugar, vanilla, cinnamon and salt. Pour evenly over bagels.
Press bread gently so it absorbs custard. Cover and refrigerate 2 hours, or overnight.
Heat oven to 350 F.
To make crumble topping: Mix flour, brown sugar, cinnamon, salt and melted butter. Sprinkle mixture over casserole. Add remaining berries.
Bake 40-45 minutes until center is set and top is light golden color. If it browns too quickly, tent loosely with foil.
Let rest 5-10 minutes. Dust with powdered sugar and serve with maple syrup. Top with extra berries, if desired.