Preheat the oven to 425ºF. Use a zester, microplane, or small-holed cheese grater to remove the zest from the lemon.
Place the thawed berries in a bowl and add the sugar, cornstarch, about ½ tsp of the zest, and 1 Tbsp of the lemon juice. Stir to combine. Pour the prepared berries and all the juices into a small casserole dish (4 cup capacity).
In a medium bowl, stir together the flour, baking powder, sugar, salt, and another ½ tsp of the lemon zest. Cut the butter into small pieces, then add it to the flour mixture. Use your hands to work the butter into the flour until the butter is in very small pieces and the mixture resembles damp sand. Add the milk and stir until a slightly sticky dough forms.
Drop the biscuit dough onto the berries in small pieces. It's okay if the dough does not completely cover the berries.
Bake the cobbler in the preheated oven for 20-25 minutes, or until the top is golden brown and the berries are bubbling up around the edges. To prevent messes from the berry juices bubbling over, place the casserole dish on a baking sheet covered with parchment to catch any spills.
Serve warm topped with ice cream or whipped cream, if desired. Garnish with any remaining lemon zest.