Preheat the oven to 350°F (175°C). Grease a 9x5-inch loaf pan or line it with parchment paper for easy removal.
In a large mixing bowl, whisk together the flour, baking powder, baking soda, salt, and sugar until evenly combined and no lumps remain.
Add the chopped cranberries and chopped walnuts to the flour mixture. Stir well so the fruit and nuts are lightly coated with flour. This helps prevent them from sinking while the bread bakes.
In a separate bowl, whisk together the egg, buttermilk, vegetable oil, vanilla extract, and orange zest until the mixture is smooth and fully combined.
Pour the wet ingredients into the dry ingredients. Gently stir with a spatula or wooden spoon until just combined. The batter should be thick and slightly lumpy. Do not overmix.
Transfer the batter to the prepared loaf pan. Spread it evenly and lightly smooth the top.
Bake for 50 to 60 minutes or until a toothpick inserted into the center comes out clean or with a few moist crumbs attached.
Remove the loaf from the oven and allow it to cool in the pan for 10 minutes.
Carefully transfer the bread to a wire rack and let it cool completely before slicing and serving.