Preheat oven to 350°F. Line a 9x5-inch loaf pan with parchment paper, allowing parchment to hang over the long edges of the pan. Grease well with softened or melted butter. Set aside.
In a medium bowl, whisk together flour, baking powder, baking soda, salt, and (optional) cinnamon. Set aside.
In a large bowl, mix together melted butter and sugar. Add in eggs and vanilla. Mix until eggs are well combined into mixture. Add banana and mix until incorporated.
Add half of dry flour mixture. Stir until almost combined. Add sour cream. Mix until almost incorporated.
Add half of the remaining dry flour mixture. Stir until almost combined. Toss the remaining flour mixture with the strawberries and walnuts. Add to batter. Fold to combine until strawberries and walnuts are evenly distributed. *DO NOT overmix. It's okay if these are still slight streaks of flour in the batter. Transfer batter to prepared loaf pan. Smooth top into an even layer. If desired, arrange fresh strawberry slices on top of batter.
Bake for 60-70 minutes until toothpick inserted in center of loaf comes out clean. Cool banana bread in pan for 10 minutes before unmolding. Cool to room temperature on wire rack before slicing.