Preheat broiler to high and position rack to highest level.
Wash, pat dry, and place all berries in a saute pan that fits them tightly in 1 layer and can also be used for serving. Sprinkle berries with 1 tablespoon sugar substitute, and then cover them with a thick, even layer of sour cream, letting the berries peek out around the edges of the pan. Sprinkle the remaining 1/4 cup sugar substitute evenly over the top, and then sprinkle the brown sugar substitute over top.
Place the pan as close under the broiler as you can while keeping the oven door open and the handle of the pan sticking out. The berries only need to be warmed and the sugar substitute glazed. Broil for 2 to 3 minutes, or until just caramelized, watching carefully the whole time. Serve warm as you would a cobbler, and garnish with fresh mint.