Coat a 9x13-inch casserole dish with nonstick spray. Arrange the bread cubes (see notes if the cubes are still very soft or fresh) in an even layer in the pan, then sprinkle the blueberries over the top. Set aside.
In a large mixing bowl, whisk together the eggs, almond milk, maple syrup, lemon zest, lemon juice, vanilla, cinnamon, and salt. Pour over the bread. Cover the pan tightly with plastic wrap and place in the refrigerator overnight. (If overnight is not possible, I recommend a minimum of 2 hours).
When ready to bake, remove the pan from the refrigerator. Place a rack in the center of your oven and preheat the oven to 350 degrees F. Bake, uncovered, for 45 to 55 minutes, until lightly golden brown on top and the bread in the center is moist but no longer liquidy. Serve hot, dusted with powdered sugar and topped with butter and maple syrup, as desired.
*If time allows, I recommend setting the cubes out overnight to dry out so that they absorb the max amount of custard. If your bread is still fresh or very soft, toast it in the oven first: Place two racks in the upper and lower thirds of your oven and preheat the oven to 350 degrees F. Spread the cubes out in a single layer on a large baking sheet. Bake for 10 minutes, until lightly dried, tossing once halfway through. Use in the recipe as directed.
TO STORE: Cover and refrigerate casserole for up to 4 days.
TO REHEAT: Rewarm leftovers in a baking dish in the oven at 350 degrees F.
TO FREEZE UNBAKED: Prepare the recipe up to step 2, but do not refrigerate overnight. Instead, place in the freezer and freeze for up to 2 months. Thaw overnight in the refrigerator, then bake as directed.
TO FREEZE BAKED: Let cool completely, cover tightly, and freeze up to 3 months. Let thaw overnight in the refrigerator and warm up in a 350 degree F oven until heated through, about 30 to 40 minutes. If the French toast starts to become too brown while you are reheating it, cover with foil, then continue baking until hot.