Mix the 2 teaspoons of sourdough starter with 1/4 cup water and 1/2 cup flour in a small bowl until combined. Cover loosely and let sit at room temperature (around 78°F) overnight or for 8-12 hours until the mixture is bubbly and active. This creates your levain, which will be the foundation of your dough's flavor and rise. You'll know it's ready when it has risen and shows visible bubbles throughout.

In a large mixing bowl, combine the bubbly levain from Step 1 with 1.9 cups water, stirring until mostly dissolved. Add 3 tablespoons sugar, 2 teaspoons salt, and 2 tablespoons lemon zest, mixing well to distribute the zest evenly throughout. Add 4 cups flour and mix until a shaggy, sticky dough forms—don't worry about it being messy at this stage. Cover with a damp towel and let rest for 30 minutes (this is your autolyse, which allows the flour to fully hydrate and develops gluten naturally).

Perform 4-6 coil folds using wet hands: reach down to the bottom of the dough, stretch it up and over itself, rotating the bowl as you work around the dough. This takes about 1-2 minutes total. Cover and rest for 30 minutes at room temperature. Repeat this folding and resting process two more times (so three sets of folds and rests total, 30 minutes each). These gentle folds build strength and gluten structure without aggressive kneading. I prefer coil folds over stretch-and-folds for sourdough because they're less likely to deflate the air bubbles that have already formed.

After your final coil fold, gently fold 1.25 cups fresh blueberries into the dough, being careful not to crush them too much—you want some to remain whole for texture. Cover and let the dough rise at room temperature (78°F) for 2.5-3 hours until it has increased in volume by about 50% and shows visible bubbles under the surface. The dough should jiggle slightly when you gently shake the bowl but shouldn't be completely airy yet—this is bulk fermentation.

Line a 9x13-inch baking pan (or similar size) with parchment paper and coat generously with 3 tablespoons olive oil. Gently transfer the dough to the prepared pan and use oiled fingers to slowly stretch and spread it to fill the pan evenly, working from the center outward. Don't force it—if it springs back, let it rest for 5-10 minutes before continuing. Cover loosely with plastic wrap or a towel and let rise at 78-80°F for 2-3 hours until the dough is puffy and springs back slowly when poked (it should leave a slight indentation).

While the dough is in its final rise, prepare the crumb topping by mixing 1/2 cup flour, 2.5 tablespoons brown sugar, 1/4 teaspoon salt, and 3/4 teaspoon cinnamon in a small bowl. Cut 3 tablespoons cold unsalted butter into 1/4-inch cubes and work it into the dry mixture with your fingertips until it resembles coarse breadcrumbs. When the dough is ready to bake, drizzle 2 tablespoons melted butter evenly over the surface. Sprinkle the crumb topping and 1/3 cup fresh blueberries over the dough, pressing them gently into the surface. Use your fingertips to create dimples all over the focaccia—these wells help distribute toppings and create the characteristic focaccia texture.

Preheat your oven to 425°F and bake the focaccia for 25-30 minutes until the top is deep golden brown and the internal temperature reaches 200°F when measured in the center. While the focaccia bakes, whisk together 1 cup sifted powdered sugar, 4 teaspoons fresh lemon juice, 1 teaspoon vanilla, and a pinch of salt until you have a pourable glaze—it should be thick but smooth. When the focaccia comes out of the oven, let it cool for just 2-3 minutes, then pour the glaze evenly over the warm bread. The warmth helps the glaze set beautifully. I always use freshly squeezed lemon juice rather than bottled because the acidity level is more predictable and gives you a brighter flavor against the sweet glaze.