Preheat your oven to 400°F and line a baking sheet or cast-iron skillet with parchment paper.
In a large mixing bowl, combine the dry ingredients: purpose flour, whole wheat flour, baking soda, kosher salt, and caraway seeds.
In a separate bowl, mix the wet ingredients: sourdough discard, buttermilk, sugar, lemon juice, 1 egg, orange zest and melted Irish butter.
Pour the wet ingredients into the flour mixture and stir using a wooden spoon or dough whisk until a sticky dough forms. Add raisins.
Turn the dough onto a well floured surface and gently shape it into a round ball.
Transfer the dough to the prepared baking sheet, cast-iron skillet, or loaf pan.
Using a sharp knife, cut a deep cross into the top of the bread.
Beat the remaining egg yolk and brush it over the top of the dough.
Bake for 30–35 minutes or until the loaf is golden brown and cooked through.
Allow the bread to cool slightly before slicing.