Lightly grease a large, oven-safe baking dish.
In a medium mixing bowl, whisk together the ground flaxseed and filtered water. Let stand for 5 to 10 minutes to thicken.
Meanwhile, add the bread cubes to a large mixing bowl.
To the flaxseed mixture, add the almond milk, apple sauce, coconut milk, maple syrup, vanilla, nutmeg, cinnamon, and sea salt. Vigorously whisk until smooth.
Pour the mixture over the bread cubes, and thoroughly toss to coat, ensuring they're all evenly covered.
Spread the coated bread cubes evenly into the prepared baking dish. Cover and refrigerate for at least 8 hours or overnight (can be refrigerated for up to 24 hours before baking). Note: At this time, you'll also want to soak the cashews for the crème anglaise—see recipe here.
In the morning (or just before baking), preheat the oven to 350F.
Uncover the dish and bake for 45 minutes to 1 hour, or until set and desired doneness is reached. (I prefer a slightly crisp top with a soft, slightly gooey bottom—almost like a bread pudding—so I bake mine for 45 minutes, but baking time will depend on taste preferences as well as the type of bread you use.)
Remove the casserole from the oven and cool for 10 minutes before serving.
Serve with generous spoonfuls of spiced cashew crème anglaise and drizzles of maple syrup.