Add the frozen berries to a large nonstick pot over medium heat. Add in the sugar, lemon juice, cornstarch, salt, and fresh mint and stir to combine.
Once the fruit begins to break down, reduce the heat to medium-low and cook gently for 10-15 minutes stirring frequently. Once the berries begin to break down and the filling starts to thicken, remove from the heat.
Preheat the oven to 375°F.
In a large bowl, whisk together the flour, sugar, baking powder, and salt.
Cut the cold butter into the dry ingredients by flattening the cubes into disks. You want the butter to remain in relatively large chunks (about quarter sized).
Slowly drizzle the cream over top, combining with a fork as you go. Using a spatula or bowl scraper, fold the dough over onto itself a few times in the bowl until it comes together into a sticky dough.
Pour the mixed berry filling into a baking dish. Then dot the topping all over the berries until they're mostly covered.
Before baking, lightly brush the top of the biscuits with cream and sprinkle with sparkling sugar if desired.
Bake at 375°F for 30-40 minutes until the filling is bubbly and the top is golden brown.
Serve the mixed berry cobbler warm with vanilla ice cream or homemade whipped cream!
