Prepare the casserole the night before you want to bake and serve it.
Butter a 9x13 baking dish very well.
Tear or cut the stale bread into rough, 2-inch chunks and line the pan with the bread.
Crack the eggs into a large mixing bowl. Whisk very well. Add the remaining french toast ingredients to the bowl, and whisk to combine.
Pour the mixture over the bread. If needed, press down gently to make sure all the bread is in contact with the egg mixture.
Cover and refrigerate overnight.
Prepare the topping by combining all the ingredients in a small bowl with a pastry cutter or your fingers, until it resembles coarse crumbs. Cover and store in the fridge.
The next morning, remove the casserole from the fridge while the oven preheats to 350F. Tip the pan to one side. If you notice a lot of liquid, drain the excess liquid from the casserole (this varies depending on the bread).
Sprinkle the streusel topping evenly over the casserole.
Once the oven is hot, bake the casserole for 50-65 minutes (depending on the pan used--dark ceramic pans usually require more baking time than clear glass pans), until set. The casserole is done when the center is set.
Let the casserole cool for a few minutes (about 5-10 minutes), then slice and serve with maple syrup (if desired).
Notes
*Spelt, all-purpose, or wheat flour may be used in place of the einkorn flour. Oat flour may also work.