Preheat the oven to 350°F (175°C). Grease the loaf pan or line it with parchment paper.
In a large mixing bowl, whisk together the all-purpose flour, granulated sugar, baking powder, baking soda, and salt.
In another bowl, combine the melted unsalted butter, eggs, orange zest, and fresh orange juice. Mix until well blended.
Pour the wet ingredients into the dry ingredients, and stir gently until just combined. Do not overmix; it's okay if there are a few lumps.
Gently fold in the halved cranberries and chopped walnuts if using.
Transfer the batter to the prepared loaf pan, smoothing the top with a rubber spatula.
Bake in the preheated oven for 55-60 minutes, or until a toothpick inserted into the center comes out clean.
Once baked, remove the bread from the oven and let it cool in the pan for 10 minutes. Then, transfer it to a wire rack to cool completely.