Preheat the oven to 350F. Line a 5" x 10" loaf pan with parchment paper or lightly grease it with oil.
In a medium mixing bowl, whisk together the ground flaxseed and water. Let stand for 5 minutes to thicken. Then, whisk in the remaining wet ingredients (i.e., sweet potato purée, carrot, grapeseed oil, coconut sugar, maple syrup, and vanilla).
In a large mixing bowl, whisk together the dry ingredients (i.e., spelt flour, cinnamon, nutmeg, baking soda, baking powder, and sea salt).
Add the wet mixture to the dry mixture and use a large wooden spoon to stir until just combined (read: no flour patches remain). Be careful not to overmix—spelt has a delicate gluten structure, so overmixing will result in dry, crumbly bread.
Spoon the batter into the lined loaf pan and use a spatula to gently smooth the surface.
If desired, garnish the top with a light sprinkling of pepitas and/or walnuts. Press down lightly to ensure they stick to the batter.
Bake for 45 to 55 minutes, or until a toothpick inserted into the center comes out clean with a few crumbs at the tip.
Cool for 15 minutes in the pan, and then very carefully transfer to a cooling rack to cool completely.
Cover and store in the refrigerator for up to 1 week or freeze for up to 1 month.