Preheat a grill on medium heat.
Spread the mascarpone on the bottom half of the croissant. Spread the jam on the top half. Evenly space the berries over the cheese, then place the top on,
Turn the heat to low, and place the sandwich on the grill. Grill for 2-3 minutes, checking to make sure that the bottom is not burning. Flip the sandwich and repeat for another 2 minutes. Remove and serve hot.