Preheat oven to 375°F (190°C).
Make the berry filling. In a large bowl, combine the berries, brown sugar, flour and salt. Gently stir to combine. Add the zest and juice of the lemon. Toss to combine. Transfer to an 8" x 8" baking dish or a 10-12" cast iron skillet. Any vessel that is approximately these sizes and oven proof will work.
Make the crisp topping. Wipe out your bowl. Combine the flour, cornflakes (reserve about ⅓ of the cornflakes), brown sugar, cardamom and salt. Stir to combine. Cut the cold butter into ½" pieces. Add the butter to the dry ingredients. Rub the butter into flour, using your fingertips and thumbs. Some of the cornflakes will get crushed, which is fine. Once the mixture is crumbly and able to hold together in clumps when squeezed, you're good.
Add the remaining cornflakes and gently toss into the topping, taking care not to crush them. Pile the crisp topping onto the berries.
Bake until the fruit is bubbly and the topping is golden brown, about 45-50 minutes. Serve warm with vanilla ice cream, whipped cream, custard or cream.