At 10:00 PM, whisk the sourdough starter into the water in a small bowl. Mix in the bread flour until well combined. Knead briefly to form a smooth ball, then shape it into a cylinder. Place it in a tall glass jar and cover it completely with room-temperature water. Let it sit overnight for 8 to 10 hours.

At 8:00 AM, carefully remove the floating starter from the water. Discard the wet outer layer and measure out 60g from the firm center for your dough. In a stand mixer bowl, combine the extracted starter, milk, eggs, bread flour, and sugar. If using, add the optional yeast. Mix briefly until a shaggy mass forms, then cover and let it rest for 30 minutes.

At 8:30 AM, attach the dough hook and mix on low speed for 2 to 3 minutes until well combined. Add the salt and continue mixing for another 2 minutes until a clean ball forms. Gradually add the soft butter, one tablespoon at a time, and knead for 10 to 15 minutes until the dough is smooth and elastic. If the dough is too sticky, add a little more flour as needed.

At 9:00 AM, cover the bowl tightly and let the dough proof for 3 to 4 hours at a warm temperature. During this time, perform two rounds of stretches and folds to help the dough rise.

At 12:00 PM, transfer the covered dough to the refrigerator for a long cold fermentation until the next morning.

At 7:00 AM, turn the cold dough out onto a work surface and divide it into 10 equal pieces, each weighing 60g. Shape each piece into a ball, cover, and let them rest for 15 minutes.

For the filling, chop the strawberries and rhubarb into small pieces. In a small bowl, mix the sugar with the rice starch until well combined.

Roll each dough ball into a thin circle. Place a generous spoonful of fruit in the center and sprinkle with a teaspoon of the sugar-starch mixture. Fold the dough over the filling and pinch the edges to seal tightly. Place the piroshki seal-side down on a parchment-lined tray, leaving space between them. Cover loosely and let them proof for 2 to 4 hours until they are puffed up.

Preheat the oven to 375°F (190°C). Brush the tops of the piroshki with an egg wash. Bake in the preheated oven for 10 minutes to set the oven spring, then reduce the temperature to 350°F (175°C) and continue baking for another 10 to 15 minutes until golden brown.

While the piroshki are baking, mix the warm water with the sugar until dissolved. Once the piroshki are out of the oven, brush the sweet syrup over the tops. Allow them to cool completely before serving.