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., butternut squash purée, carrot, grapeseed oil, coconut sugar, maple syrup, and vanilla).
Stir in the sea salt, baking powder, baking soda, nutmeg, cinnamon, and spelt flour into the wet mixture, one by one, in the order listed. Stop stirring when there are no flour patches at the bottom of the bowl. Be careful not to overmix—spelt has a delicate gluten structure, so overmixing will result in dry, crumbly bread. The batter should be quite thick and fluffy.
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, hemp seeds, and/or walnuts. Press down lightly to ensure they stick to the batter.
Bake for 50 to 65 minutes, or until a toothpick inserted into the center comes out clean.
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.