Mix the lemon juice with the milk and let it sit.
Optional: Preheat the oven to 200ºC (392ºF) and toast the walnuts for about 10 minutes until golden. Reduce the oven temperature to 180ºC (356ºF).
Set one banana aside. Mash the remaining bananas with a fork until you get a smooth purée.
Beat the butter and sugar until creamy.
Add the eggs one at a time, mixing after each addition.
Mix in the vanilla, mashed banana, and buttermilk, mix again.
Add the dry ingredients and mix just until incorporated.
Gently fold in the cooled, roughly chopped walnuts.
Pour the batter into a greased and parchment-lined rectangular loaf pan.
Slice the reserved banana lengthwise and place it on top of the banana bread before baking. Sprinkle about 2 teaspoons of brown sugar on top.
Bake for about 60 to 70 minutes. Halfway through baking, cover the top with parchment paper to prevent it from burning.
Let it cool completely in the pan before removing. Serve plain or with any desired toppings (I used a bit of whipped cream and red fruits).