Lightly coat a 9-by-13-inch baking pan with vegetable cooking spray. Set aside.
Cut the bread into 1-inch pieces. Layer 1/3 of the bread cubes in the prepared pan. Top with 1 cup of blueberries. Repeat with half the remaining bread, then another cup of blueberries. Finally, scatter the remaining bread cubes on top and then 1 more cup of blueberries. Set aside.
In a large mixing bowl with a spout, whisk the eggs until blended. Add the milk, melted butter, sugar, vanilla, salt, cinnamon and lemon zest; whisk to combine.
Pour the egg mixture evenly over the bread cubes and blueberries. Cover the casserole with a piece of foil that has been lightly coated with cooking spray (sprayed side down). Place another heavy 13-by-9-inch casserole dish on top of the foil as a weight. Seal the edges of the foil around the top of the pan. Refrigerate for at least 8 hours or up to 24.
When ready to bake, preheat oven to 350F. Remove the top baking dish. Adjust the foil so it’s not sitting on top of the bread and re-seal around the edges. Allow the casserole to sit at room temperature while preheating the oven.
Bake the casserole, covered, for 45 minutes. Remove the foil, sprinkle with coarse sugar and almonds if using, and continue baking until golden brown, about 25 minutes more. Remove from the oven and top with the remaining 1 cup of blueberries. Allow the casserole to cool for 10 minutes before serving.
Top each serving with powdered sugar and/or syrup.