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 the baking dish.
To the flaxseed mixture, add the sweet potato purée, coconut milk, almond milk, maple syrup, vanilla, cinnamon, nutmeg, and sea salt. Vigorously whisk until smooth.
Pour the sweet potato mixture over the bread cubes, ensuring they're as evenly covered in the mixture as possible. If needed, use a spatula to nudge them around the baking dish a bit.
Cover and refrigerate for at least 12 hours or overnight (can be refrigerated for up to 24 hours before baking).
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 and soft, slightly gooey bottom, so I bake mine for exactly 45 minutes but baking time will vary based on preferences and the type of bread you use.)
Remove from the oven and cool for 10 minutes before serving.
Serve with generous spoonfuls of warmed Hero fruit spread and drizzles of maple syrup.