Mix the dough: In a large mixing bowl, combine the water and active sourdough starter. Break up the starter with a fork or Danish dough whisk until mostly dissolved. Add the milk, oil, and honey and mix well. Add the bread flour, whole wheat flour, and salt and stir until a shaggy, sticky dough forms and no dry flour remains.
Rest: Cover the bowl and let the dough rest at room temperature for 30 to 60 minutes.
Stretch and folds: With wet hands, perform 3 to 4 rounds of stretch and folds (or coil folds) every 15 minutes.
Bulk fermentation: Cover and bulk ferment at room temperature (or transfer to the refrigerator to slow the fermentation) until the dough is puffy and doubled in volume. *See detailed notes about bulk fermentation times in the post.
Prepare filling: The next day, soak raisins in hot water for 5 to 10 minutes if desired. Drain and pat dry. In a small bowl, mix the brown sugar and cinnamon together.
Add raisins: Turn the dough out onto a lightly floured surface and gently press into a large rectangle. Cover half of the dough with raisins, fold the other half over, and press to seal. Turn the dough 90 degrees and repeat with more raisins, folding and sealing again.
Stretch dough: Turn the dough 90 degrees again and gently stretch into a long, skinny rectangle. Use a rolling pin if needed.
Add cinnamon sugar: Spread the cinnamon sugar mixture evenly over the dough, leaving a 1/2-inch border at one far short end.
Roll into loaf: Starting at the short end closest to you, roll the dough tightly into a log shape. Pinch to seal the seam, bottom, and sides.
Pan the loaf: Place the dough seam-side down into a well-greased pullman loaf pan or loaf pan. Cover loosely with plastic wrap or a proofing bag.
Final proof: Proof at room temperature until the dough fills the pan and rises close to the rim.
Bake: Preheat the oven to 375°F about 30 minutes before the loaf is finished proofing. Bake for 45 to 50 minutes until dark golden brown and the internal temperature reaches 190°F to 200°F.
Cool: Remove the loaf from the pan immediately. Brush the top with melted butter or coconut oil while warm for a soft crust. Cool on a wire rack for at least 1 hour before slicing.