DAY 1: Soak the Raisins and Make the Preferment: Place the raisins in a small bowl, pour cold water over them, cover and set aside to hydrate overnight.

In a small bowl, combine the sourdough culture, flour, and water and mix until no dry patches of flour remain. Cover and let stand at room temperature overnight.

DAY 2: Make the Dough: In a stand mixer bowl, combine the preferment, flour, granulated sugar, salt, yeast, egg, and milk. Mix on medium-low speed until you have a firm yet tacky dough, then increase speed to medium and continue mixing until smooth and elastic.

Drain the raisins. With the mixer running, add the butter one piece at a time until fully incorporated. Continue mixing until the dough pulls away from the sides of the bowl and is soft, smooth, and elastic.

Add the raisins and mix until evenly distributed. Cover and let rise at room temperature until doubled in size.

Make the filling: In a small bowl, combine the brown sugar, flour, and cinnamon.

Grease an 8½×4½-inch loaf pan.

Shape the dough and let it rise: Lightly flour a work surface, roll the dough into a 15×8-inch rectangle, brush with egg wash, and sprinkle the filling evenly over the dough.

Fold the edges in, roll the dough into a log, pinch to seal, and place seam side down in the prepared pan. Cover and let rise until it crowns about 1 inch over the rim of the pan.

Preheat the oven to 350°F.

Bake: Brush the top of the loaf with reserved egg wash and sprinkle with turbinado sugar. Bake until golden brown and internal temperature reaches 190°F.

Remove from the oven, turn out of the pan onto a rack, and let cool completely before slicing.