With oven rack in middle position, preheat to 350°F. Grease a 9 x 5 loaf pan and line bottom with parchment paper, then grease paper. Mix together milk and lemon juice. Let stand until milk curdles, 1 minute.
Mix flour, baking powder, salt, and baking soda in a bowl. In a large bowl, beat butter, yogurt and sugar with electric mixer at high speed until fluffy, approx. 2 minutes. Add eggs, 1 at a time, beating until combined, and then mix in bananas until combined. (Mixture looks curdled.) Add flour mix to banana mix adding milk alternately, mixing at low speed until batter is smooth. Stir in walnuts or whatever is preferred.
Pour into loaf pan and bake until toothpick in center of bread comes out clean, approx. 1 hour. Cool bread in pan for 20 minutes, then remove from pan and place bread on rack. Remove paper and turn bread over to cool.