In a medium saucepan, combine gooseberries, granulated sugar, and water. Bring to a simmer over medium heat, stirring occasionally, until gooseberries begin to soften and burst, about 8-10 minutes.

In a small bowl, whisk together cornstarch with 2 tablespoons of cold water to create a slurry. Stir the slurry into the simmering gooseberry mixture. Cook, stirring constantly, until the filling thickens, about 1-2 minutes.

Remove from heat and stir in lemon zest and vanilla extract. Pour the gooseberry filling into a 9x13 inch baking dish, spreading evenly. Set aside.

Preheat oven to 375°F (190°C).

In a large mixing bowl, whisk together flour, baking powder, baking soda, and salt.

Add the cold, cubed butter to the dry ingredients. Use a pastry blender or two forks to cut the butter into the flour mixture until it resembles coarse crumbs with some pea-sized pieces of butter remaining.

Pour in the buttermilk and stir with a spoon or your hands until just combined. Do not overmix; the dough will be slightly shaggy.

Turn the dough out onto a lightly floured surface. Gently pat or roll the dough to about 1/2 inch thickness. Using a 2-inch biscuit cutter, cut out biscuits. Rework scraps gently once if needed to get more biscuits. You should get about 10-12 biscuits.

Arrange the cut biscuits evenly over the hot gooseberry filling in the baking dish. You can place them close together as they will expand.

Optionally, sprinkle 2 tablespoons of granulated sugar over the tops of the biscuits for extra sparkle and sweetness.

Bake for 30-40 minutes, or until the biscuits are golden brown on top and cooked through, and the gooseberry filling is bubbly around the edges.

Remove from oven and let cool slightly on a wire rack while you prepare the glaze.

In a small bowl, whisk together powdered sugar and lemon juice until smooth. Add milk or water one teaspoon at a time until the glaze reaches a pourable consistency.

Drizzle the lemon glaze over the warm biscuit casserole. Serve warm and enjoy!