1. PREPARE PAN AND MIX DRY INGREDIENTS: Preheat the oven to 325°F. Grease a 12-inch tube pan very thoroughly with butter or nonstick spray, reaching into every crevice. Whisk together the 3 cups of flour, baking soda, salt, and lemon zest in a medium bowl until well combined — the zest distributes evenly through the dry ingredients, which helps it disperse uniformly throughout the batter. 

2. CREAM BUTTER AND SUGAR: Beat the room-temperature butter and granulated sugar together on medium speed for 3–4 minutes until the mixture turns pale, light, and significantly increased in volume. Proper creaming creates the structure the heavy batter needs to stay tender rather than dense and gummy.

3. ADD EGGS ONE AT A TIME: Add the room-temperature eggs one at a time, mixing on low to medium speed after each addition until fully incorporated before adding the next one. After all the eggs are in, the batter should look smooth and creamy. Rushing this step can break the batter.

4. ALTERNATE FLOUR AND SOUR CREAM: Add half of the flour mixture to the batter and mix on low speed just until combined. Add half of the sour cream and mix just until blended. Repeat with the remaining flour, then the remaining sour cream, mixing each addition just until it disappears into the batter — do not overmix. Add the fresh lemon juice and lemon extract and mix briefly until just combined.

5. ADD BLUEBERRIES AND FILL THE PAN: Toss the washed, slightly damp blueberries in the reserved 1 tablespoon of flour until each berry has a light coating. Optional but recommended: spoon a thin layer of plain batter — about 1/2 cup — into the bottom of the prepared tube pan before folding in the blueberries. This base layer catches berries that sink. Fold the flour-coated blueberries gently into the remaining batter with a rubber spatula using as few strokes as possible. Pour the batter into the prepared pan and smooth the top.

6. BAKE LOW AND SLOW: Bake at 325°F for 1 hour and 15 minutes, or until a toothpick inserted into the thickest part comes out clean. Start checking at the 1-hour mark — the top should be deep golden and the cake should pull slightly away from the sides. Remove from the oven and immediately run a thin knife around both the inner tube and the outer edge to loosen the cake. Cool in the pan for 10 minutes, then invert onto a wire rack to cool completely.

7. MAKE THE GLAZE AND FINISH THE CAKE: Once the cake has cooled completely, whisk together the powdered sugar, fresh lemon juice, and milk until the glaze is smooth and pourable — it should run off the whisk in a slow, steady ribbon. Add another tablespoon of milk if it seems too thick. Drizzle the glaze over the top of the cooled cake and let it run naturally down the sides. Immediately sprinkle the lemon zest from the half lemon over the wet glaze. Let the glaze set for 20–30 minutes before slicing and serving.