At 10:00 PM, mix the 10g of sourdough starter with 30g of water in a small bowl. Add 60g of bread flour and stir well. Knead briefly to form a smooth ball, then shape it into a firm cylinder. Place this cylinder in a tall glass jar and cover it completely with room-temperature water (74–78°F or 23–26°C). Let it sit overnight for 8 to 10 hours. It will expand and float to the top as it ferments.

At 8:00 AM, carefully remove the floating starter from the water. Discard the wet outer layer and weigh out 60g from the firm center for your dough. In your stand mixer bowl, combine the 60g of stiff starter, 150g of milk, 50g of eggs, 300g of bread flour, and 75g of sugar (add the optional 0.5g of dry yeast if desired). 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 the dough on low speed for 2 to 3 minutes (or on speed 3 for 3 to 4 minutes in a KitchenAid mixer) until well combined. Add the 6g of salt and continue mixing for another 2 minutes until a clean ball forms. Increase the mixer speed to medium-high and gradually add the 60g of soft butter, one tablespoon at a time. Knead for 10 to 15 minutes until the dough is strong enough to clear the sides of the bowl. If the dough is too sticky, you can add a little more flour (20g to 40g).

At 9:00 AM, cover the bowl tightly and let the dough proof for 3 to 4 hours at a warm temperature (76–80°F or 24–28°C). During this time, perform two rounds of stretching and folding to help the dough become light and slightly puffy.

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 tight ball, cover them, and let them rest for 15 minutes.

For the filling, chop your strawberries and rhubarb into small pieces. In a small bowl, mix 6 to 8 tablespoons of sugar with 1 tablespoon of 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 1 teaspoon of the sugar-starch mixture. Fold the sides of the dough over the filling and pinch the edges together to seal securely. Place each piroshki seam-side down on a parchment-lined tray, leaving about 1 inch between them. Cover loosely and let them proof on the counter for 2 to 4 hours until they are visibly larger and puffier.

Preheat your 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 an additional 10 to 15 minutes until they are golden brown.

While the piroshki are baking, prepare the sweet glaze by mixing 6 tablespoons of warm water with 3 tablespoons of sugar until dissolved. Once the piroshki are out of the oven, brush the sweet syrup over the hot tops. Allow them to cool completely before serving.