Prep the Pan & Oven: Preheat your oven to 350°F (175°C). Grease and flour a 9x5-inch loaf pan, or line it with parchment paper for easy removal.
Toast the Walnuts: Spread the chopped walnuts on a small baking sheet. Toast in the preheated oven for 5-7 minutes, or until they are fragrant. This step is optional but highly recommended for a richer flavor. Let them cool completely.
Combine Dry Ingredients: In a medium bowl, whisk together the all-purpose flour, baking soda, salt, cinnamon, and nutmeg. Set aside.
Mix Wet Ingredients: In a large bowl, using an electric mixer or a whisk, cream together the softened butter, granulated sugar, and brown sugar until light and fluffy.
Add Eggs & Wet Ingredients: Add the eggs one at a time, mixing well after each addition. Beat in the vanilla extract, Greek yogurt, and mashed bananas until just combined. The mixture may look a little curdled, but that's okay.
Combine & Fold: Add the dry ingredients to the wet mixture and mix with a spatula until just combined. Do not overmix! Overmixing will develop the gluten and result in a tough, dense bread. Gently fold in the toasted walnuts.
Bake: Pour the batter into your prepared loaf pan and smooth the top with a spatula. Bake for 55-65 minutes, or until a toothpick inserted into the center comes out with only a few moist crumbs attached.
Cool & Slice: Let the banana bread cool in the pan for 10-15 minutes before transferring it to a wire rack to cool completely. Slicing it while it's still warm will cause it to fall apart.