Grease a 9x13 pyrex pan with butter
Cut the brioche bread into cubes and spread into the greased baking pan.
Pour the blueberries and the lemon and orange zest over the bread. Reserve some of the zest for garnish after the casserole is baked.
In a medium bowl mix together the eggs, milk, cinnamon, and vanilla. Pour evenly over the bread pieces.
Let the bread soak up all the custard. If making overnight, cover the casserole with plastic wrap and refrigerate. Tip: I like to set a small dish inside the pyrex baking dish on top of the plastic covering to push the bread down into the custard mixture to fully soak the top bread pieces.
If you are making this the same day, preheat the oven to 350, allow the bread mixture to soak up all the liquid for about 20 minutes while you make the topping.
For the topping combine flour, sugar, and cinnamon in a small bowl. Add the cold cubed butter and cut into the butter with a fork or pastry cutter until it's incorporated into the flour mixture and is crumbled.
Pour the strudel topping evenly over the casserole and bake for 50 minutes until the top is golden brown and the custard has set.
Garnish with reserved orange zest. Cut into 2 x 2 pieces and serve with warm maple syrup.