Bloom the yeast. In the bowl of a stand mixer, whisk together the warm milk (180 g / ¾ cup) and instant yeast (7 g / 2¼ tsp). Let sit 5 minutes until foamy.

Mix the dough. Add the sourdough discard (240 g / 1 cup), sugar (50 g / ¼ cup), softened butter (57 g / 4 Tbsp), egg, salt (7 g / 1¼ tsp), and lemon zest to the bowl. Add the bread flour (480 g / 4 cups) and mix on low with the dough hook until a shaggy dough forms.

Knead. Increase to medium speed and knead 8–10 minutes, until the dough is smooth, elastic, and pulls cleanly from the sides of the bowl (it will still stick to the bottom - that's normal for an enriched dough).

First rise. Shape the dough into a ball, place in a lightly greased bowl, cover, and let rise in a warm spot 1½–2 hours, until roughly doubled.

Make the filling. While the dough rises, toss the blueberries with sugar (67 g / ⅓ cup), cornstarch (15 g / 2 Tbsp), and lemon juice (15 g / 1 Tbsp) in a bowl. Set aside.

Roll out the dough. Turn the dough onto a lightly floured surface and roll into a 12x18-inch rectangle. Brush with the melted butter (28 g / 2 Tbsp), leaving a ½-inch border along one long edge.

Add the filling. Spoon the blueberry mixture evenly over the buttered dough, gently pressing the berries in so they don't roll out.

Roll and slice. Starting from the long edge opposite the border, roll the dough into a tight log, pinching the seam to seal. Using a bench scraper or sharp knife, slice into 12 even rolls.

Second rise. Arrange the rolls cut-side up in a buttered 9x13-inch pan, cover, and let rise 45–60 minutes, until puffy and touching.

Bake. Preheat the oven to 350°F while the rolls finish rising. Bake 28–32 minutes, until golden brown and the centers reach about 190°F.

Glaze. Let the rolls cool 10 minutes in the pan. Whisk together the powdered sugar (120 g / 1 cup), lemon juice (30 g / 2 Tbsp), lemon zest, and a pinch of salt until smooth, then pour over the warm rolls.