2 tablespoons unsalted Butter, cubed and at room temperature
Vanilla Ice Cream, for serving
For the cake batter:
Position a rack in the middle of the oven and preheat to 375 degrees. Butter the bottom and side of a 10-inch cast-iron skillet.
Whisk the flour, baking powder and salt in a medium bowl until combined. Beat the butter, sugar and lemon zest in a large bowl with an electric mixer on a medium-high speed until fluffy, about 3 minutes.
Beat in the egg until combined. Reduce the mixer to low and gradually beat in the milk.
Add the flour mixture and beat, scraping down the sides of the bowl occasionally with a rubber spatula, until smooth.
Stir in the mixed berries until evenly distributed, then spread the batter into the prepared cast iron skillet; set aside.
For the crumb topping:
Stir together the granulated sugar, brown sugar, flour, nutmeg, ginger, and salt in a small bowl until combined.
Work in the butter with your fingers until the entire mixture is moistened and clumpy. Scatter evenly over the batter.
Bake until the top is golden and an inserted toothpick comes out clean, about 1 hour.
Transfer the skillet to a wire rack and let cool at least 30 minutes before serving warm.