Pre-heat-your oven to 350 degrees. Line a loaf pan with parchment paper and set aside.
Start by making your walnut crumble. In a medium bowl, combine the flour, brown sugar, oats and walnuts and mix well. Add the solid coconut oil to use your hands to work it into the dry ingredients until the mixture is crumbly. Don’t overwork it, clumps are a good thing! Refrigerate your crumble while you make the banana bread dough.
Place four ripe bananas in a large mixing bowl. Using a potato masher or a large fork, mash the bananas well. You want them to be relatively smooth, but a few lumps are fine.
Mix the melted coconut oil into the bananas. Add the brown sugar, egg, cinnamon, nutmeg and salt, and mix well. Sprinkle the baking soda over the batter and then mix it in. Add the flour last, making sure everything is well combined.
Pour the batter into your lined loaf pan and top it with the walnut crumble in an even layer.
Bake your banana bread for 50 minutes to an hour or until a tester comes out clean.
Let your bread cool in the pan for a few minutes and then transfer it to a cooling rack. Slice before serving.