Heat oven to 350 degrees. Grease bottom only of regular loaf pan (or grease and flour bottom and sides of nonstick loaf pan); set aside.
In a large bowl combine flour, sugar, baking soda, salt, walnuts, cinnamon, nutmeg and orange zest; set aside.
In a medium bowl mix mashed bananas, yogurt, eggs, butter, and vanilla with wooden spoon. Lightly fold banana mixture into dry ingredients with rubber spatula until just combined and batter looks thick and chunky. Scrape batter into prepared loaf pan; bake until loaf is golden brown and toothpick inserted in center comes out clean, about 55 minutes. Cool in pan for 5 minutes, then transfer to wire rack. Serve warm or at room temperature.