Step 1: In a large bowl or the bowl of a stand mixer, combine the freshly milled flour, milk, sourdough starter, egg, sugar, and salt. Mix until no dry flour remains (Image 1). Cover and let the dough rest for 20-30 minutes. If the dough still feels very stiff after resting, add another 5-20 g of milk.

Step 2: Add the softened butter while mixing on low speed (Image 2). Once all the butter has been added, continue mixing on low-medium speed for about 2 minutes. Cover and let the dough rest for 30 minutes.

Step 3: Perform 2-3 sets of stretch-and-folds, spacing them about 30 minutes apart (Image 3). After the final fold, cover the dough and let it continue fermenting until it has increased by about 30-50% in volume. This usually takes 5-7 hours at 72°F (22°C). - starting from the moment the starter was added to the dough!

Step 4: While the dough is fermenting, add the blueberries, sugar, lemon zest, lemon juice, butter, salt, and vanilla to a small saucepan. Cook over medium heat for 5-8 minutes, stirring occasionally, until the blueberries have softened and released plenty of juice. (Image 4)

Step 5: Sprinkle in the cornstarch while stirring continuously. Cook for about 1 minute longer, until the filling has noticeably thickened (Image 5). If your blueberries are extra juicy, you can add up to 10 extra cornstarch! The goal is to transform the blueberry filling from liquid so syrupy. Remove it from the heat and let it cool completely before assembling the focaccia.

Step 6: Place the granulated sugar and lemon zest in a shallow bowl (Image 6). Rub them together with your fingertips until the sugar becomes fragrant and slightly moist. Pour the melted butter into a separate shallow bowl.

Step 7: Line a 9×13-inch baking pan with parchment paper (Image 7). Slice or tear off pieces of dough about 3-4 tablespoons in size (I like to do bigger ones for the first round and smaller ones for the second round). Dip each piece into the melted butter, let the excess drip off, and then coat it lightly in the lemon sugar.

Step 8: Arrange about half of the coated dough pieces in the prepared pan. Spoon about ¼ of the cooled blueberry filling between and over the pieces. (Image 8)

Step 9: Add the remaining coated dough pieces (Image 9), cover the pan and let the dough rise until it looks noticeably puffy and jiggly, about 2-3 hours.

Step 8: Preheat the oven to 400°F (200°C). Spoon over the rest of the blueberry filling (Image 10). You can drizzle any remaining melted butter over the dough too. Use your fingertips to gently press dimples across the surface without completely deflating it. Sprinkle any remaining lemon sugar over the top.

Step 11: Bake for 20-25 minutes, rotating the pan halfway through, until the top is golden brown and the dough in the center is baked through (Image 11). Let the focaccia cool completely before adding the glaze.

Step 12: Whisk together the powdered sugar, 20 g of lemon juice, lemon zest, and salt until smooth (Image 12). Add the remaining lemon juice a little at a time if you need to make the glaze thinner.

Step 13: Drizzle the glaze over the cooled focaccia (Image 13). Let it set for about 20 minutes before serving