Grease a 9-inch loaf tin with butter. Set it aside.
In a mixing bowl, combine all-purpose flour, atta, salt, and chia seeds. Set it aside.
In another large bowl, mash the bananas. Add melted butter, sugar, yogurt, vanilla extract, and lemon zest. Mix well.
Add the sourdough starter and fold it in until fully incorporated.
Add walnuts and vanilla extract. Fold them in.
Gradually add the flour mixture and mix until it comes together and all the flour is incorporated.
Pour into the prepared loaf tin. Cover loosely with a cling wrap.
Let it sit in a warm spot for 10-12 hours for fermentation. The batter should raise and increase in volume by about a half.