Preheat the oven to 375°F and generously grease a 10-inch cast-iron skillet using 1/2 tablespoon of butter. In a spacious mixing bowl, combine the flour, granulated sugar, sea salt, and baking soda, whisking until evenly distributed. Cut the cold butter into small cubes, then, using your fingertips, blend the butter into the dry mixture until it resembles coarse crumbs; fold in the raisins or cranberries, ensuring they are evenly dispersed and separated. Form a well in the center of the dry ingredients. In a separate measuring jug, whisk together the cold buttermilk and egg until smooth, then pour this mixture into the flour blend. Using a wooden spoon, stir gently just until the dough begins to come together; avoid overworking it. Turn the dough onto a lightly floured surface, and with floured hands, shape it into a rustic round loaf, handling it as little as possible to maintain tenderness. If the dough feels excessively sticky, sprinkle lightly with extra flour. Position the formed loaf into the prepared skillet and, with a sharp knife, score a large, deep 'X' across the top. Bake on the middle rack for 50–55 minutes, or until the bread sounds hollow when tapped on its base. Remove the loaf from the skillet and place it on a wire rack to cool. For best texture and flavor, enjoy within two days, storing any remaining slices refrigerated for up to one week.