Heat the oven to 375°F (190°C). Butter the dish and place it on a rimmed baking sheet. Remove stems from the blueberries.
Combine fruit sugar and cornstarch, then toss with blueberries and lemon juice. Spread evenly over the base of the dish.
Whisk topping flour, sugar, baking powder, and salt. Cut cold butter into the mixture until pea-size pieces remain.
Add milk and lemon zest. Stir just until a rough, thick dough forms without beating it smooth or working it for a long time.
Drop dough in eight mounds over the berries, leaving narrow gaps between them so you can see the filling as it cooks.
Bake 35–45 minutes, until topping is golden and a tester in a central mound shows no wet dough. Fruit should bubble near the center.
If topping is browned before the filling bubbles, cover loosely with foil and continue baking. Avoid serving with raw dough underneath the mounds.
Rest 20 minutes before spooning into bowls. Include topping and fruit in each portion, letting the thickened blueberry juices settle around the biscuit.
