1. Preheat the oven to 350°F. Grease a 9 x 5-inch loaf pan and line it with parchment paper, leaving enough overhang on the long sides to lift the bread out after baking.
2. Bloom the spice. Melt the butter and remove it from the heat. While the butter is still warm but not hot, whisk in 2 to 2½ teaspoons of the Zest of Both Worlds spice blend. Let it sit for about 5 minutes so the aromatics bloom and infuse into the butter.
3. Mix the wet ingredients. Whisk the brown sugar into the warm spiced butter, then add the eggs and whisk until well combined. Add the sour cream, mashed bananas, and orange zest, and mix until incorporated.
4. Whisk the dry ingredients. In a separate bowl, whisk together the all-purpose flour, whole wheat flour, baking soda, and salt.
5. Combine. Add the dry ingredients to the banana mixture and fold gently with a spatula just until no visible flour remains. Do not overmix. Fold in the toasted pecans, if using.
6. Rest the batter for about 5 minutes before transferring it to the pan. This lets the whole wheat flour hydrate and gives you a more tender crumb.
7. Top it. Combine the turbinado sugar, spice blend, and flaky sea salt, and sprinkle evenly over the top of the batter.
8. Bake for 55 to 65 minutes, checking at 55 minutes. A toothpick inserted in the center should come out with a few moist crumbs but no wet batter. If the top browns deeply before the center is done, tent the loaf loosely with foil for the rest of the bake.
9. Cool in the pan for about 15 minutes, then use the parchment overhang to lift the loaf onto a wire rack. Let it cool another 20 to 30 minutes before slicing.

Store in an airtight container at room temperature for up to 3 days, or wrap well and freeze for up to 2 months.