Preheat oven to 350°. Generously coat 8½x4½" loaf pan with nonstick spray (they recommend lining with parchment paper leaving generous overhand on long sides, but I didnt do this), Whisk flour, baking soda, and salt in a medium bowl. Using an electric mixer on medium-high speed, beat brown sugar, greek yogurt, and butter in a large bowl until light and fluffy, about 4 minutes. Add eggs one at a time, beating to blend. Reduce speed to low, add flour mixture, and mix until just combined. Add bananas and mix just until combined. Fold in walnuts, transfer to pan. Add sliced banana and cinnamon on top if desired. Bake bread for about 60–65 minutes. Transfer to a wire rack and let cool before enjoying.