In a large mixing bowl, combine the sourdough starter, warm milk, melted butter, sugar, eggs, and vanilla extract. Mix until smooth.
In a separate bowl, whisk together the flour, salt, cinnamon, and nutmeg. Gradually add the dry ingredients to the wet ingredients, mixing until a shaggy dough forms.
Add the currants (or dried fruit), candied peel, and citrus zest if using. Mix until evenly distributed.
Knead the dough on a lightly floured surface for about 8-10 minutes, or until the rough ball becomes smooth, elastic, and no longer sticky. Alternatively, you can use a stand mixer with a dough hook attachment on medium-low speed. Scrape down the sides of the bowl, if needed.
Once kneaded, shape the dough into a smooth ball and place it in a lightly oiled bowl. Cover the bowl with a damp cloth or plastic wrap.
Let the dough rise in a warm place for 1-2 hours, for the first rise, until it has visibly expanded. Then, refrigerate overnight for at least a 12 hour rise time, to allow the dough to ferment and develop flavor.