Preheat the oven to 180 degrees C (350 degrees F).
Line a bread loaf tin with butter paper or simply grease with butter (base 7 inches by 3 inches by 3 inches deep).
In a large bowl, mix together ½ cup wheat flour, ½ cup plain flour, ½ cup sugar, 2 tablespoon chia seeds (optional), 1 teaspoon nigella seeds, ½ teaspoon baking powder, ¼ teaspoon baking soda, ½ teaspoon cardamom powder, and ½ teaspoon salt.
In another bowl mix together, ¾ cup mashed banana (2-3 medium-size ripe bananas), 1 large egg, ¼ cup vegetable oil (any cooking oil/coconut oil), and ½ teaspoon vanilla essence.
Now pour the banana mixture into the flour bowl and mix to make a smooth and thick flowy batter.
Transfer the batter to the bread loaf tin and spread the mixture evenly. Sprinkle chopped walnuts as the top layer.
Bake for 30 to 40 minutes. You can check if the bread is ready after 35 minutes by inserting a clean knife in the center of the bread. If it comes out clean, it is fully baked. Or else you will need to keep checking after every 5 minutes.
Once baked, remove the baking tin and keep aside for cooling on a wire rack. When it is fully cooled down, it is ready to be cut into slices.
You can keep the bread at room temperature for 3 to 4 days. If you need to store it for more than that, you can refrigerate it and use it within a week.