Preheat oven to 325ºF and grease a 9×5-inch loaf pan.
Stir together bananas, apples, egg, butter, and sugar in a large bowl until just combined. In a separate bowl, mix together flour and baking soda. Stir flour and baking soda mixture into the bananas, eggs, butter, and sugar until batter is just mixed. Stir in salt.
Pour the batter into the loaf pan and sprinkle cinnamon and nutmeg over the top. Use a butter knife to swirl in the spices. Top with crushed walnuts.
Bake for 55-60 minutes, or until a knife inserted in the middle of the loaf comes out clean.