For the berry mixture: Place the berries, sugar, lemon zest, lemon juice, cinnamon, and cornstarch in a 9x9" baking dish. Stir to coat the berries evenly with the sugar; taste for sweetness and adjust. Set aside for 30 minutes.
For the topping: In a medium size bowl, whisk together 3 Tbsp. sugar, flour, baking powder, and salt.
Cut the butter in with a pastry blender or fork until the mixture resembles coarse crumbs.
With a wooden spoon, stir in the egg and milk until the batter is just moistened.
In large spoonfuls, drop the batter mixture over the berries. Bake in a preheated 350º for 30 minutes or until the berries are bubbly and the topping is nicely browned.
Serve warm with your choice of vanilla ice cream or whipped cream.