Heat the oven to 350°F. Lightly coat a loaf pan with nonstick spray and set aside.

In a large bowl, combine the flour, baking powder, salt, cinnamon, nutmeg, lemon zest, and chopped nuts. Stir until evenly mixed, then set aside.

Peel the bananas and mash them in a bowl with a fork until smooth. Measure out 1½ cups of mashed banana and transfer to a medium mixing bowl.

Add the melted (but cooled) butter, eggs, brown sugar, vanilla, buttermilk, and lemon juice to the bananas. Whisk until fully blended. Add the banana mixture to the dry ingredients and gently stir until just combined, being careful not to overmix. Fold in the blueberries, mixing only until evenly distributed.

Spread the batter into the loaf pan and smooth it. Sprinkle turbinado sugar evenly over the surface.

Bake at 350°F for 60–75 minutes, until the loaf is set. Test with a toothpick. It should have some moist crumbs, but not liquid batter. If the top browns too quickly, loosely tent the loaf with aluminum foil during baking.

Allow the bread to cool completely in the pan to keep it moist. Remove from the pan just before slicing and serving.