Preheat your oven to 350°F. Prepare a 9x5 inch loaf pan by greasing or lining with parchment paper.
In a medium bowl, whisk together the flour, nutmeg, cinnamon, baking soda, baking powder, and salt until well combined.
In a large bowl, whisk the avocado oil, sugar, brown sugar, eggs, and vanilla extract until smooth and free of lumps.
Gently stir in the mashed bananas and mashed sweet potato until evenly mixed.
Gradually fold the dry ingredient mixture into the wet ingredients using a rubber spatula. Once combined, stir in the chopped walnuts evenly.
Pour the batter into the prepared loaf pan, spreading it out evenly.
Bake for 50–60 minutes, or until a toothpick inserted into the center comes out clean. Add 5–10 minutes if needed, depending on your oven.
Remove from oven and cool in the pan for 10 minutes, then transfer to a wire rack to cool completely before slicing.