Preheat oven to 350 degrees. Line loaf pan with parchment paper and set aside.
Add flour, walnuts, baking soda, sea salt, cinnamon, ginger, nutmeg, cardamom, clove, allspice, and black pepper to a mixing bowl and stir until uniform, making sure to coat the walnuts in the dry mixture.
Peel two bananas and place into another mixing bowl. Mash bananas into a kind of puree or paste using a kitchen spoon or the back of a fork.
Add brown sugar, melted butter, milk, vanilla extract, and egg to mashed banana and mix until combined.
Add wet mixture to dry mixture and stir until just combined. Some bits of flour will remain around the walnuts and that's OK.
Transfer batter to prepared loaf pan. Peel remaining banana then cut it down the middle, lengthwise (as you would for a banana split). Lay the pieces, cut side up, on top of batter and gently press.
Place pan on center oven rack and bake for about 45 minutes or until a toothpick inserted into the center comes out clean. Cool for at least 15 minutes before removing loaf from pan.