Have 2 bowls -- one for dry ingredients, one for wet.
In the dry bowl, whisk the flour, baking soda, and salt together in a large bowl.
In the wet bowl, whisk the egg, brown sugar, and granulated sugar together until combined. Whisk in the buttermilk, oil, vanilla, and orange zest.
Pour the wet ingredients into the dry ingredients, then whisk to completely combine. Avoid over-mixing.
Fold in the cranberries and nuts (if using).
Pour the batter into bread loaf pan. Top evenly with streusel crumbles, evenly covering the bread.
Bake the bread for 45 minutes to 1 hour (mine takes exactly 50 minutes), until a toothpick inserted in the middle comes out clean.