Set a large skillet over medium heat. Add 5 plums, cut side down; sear until plums start to blister, 8 to 10 minutes. Stir plums on skin side with a wooden spoon. Sprinkle with sugar, and add one-third of the blackberries and one-third of the raspberries to the skillet.
Cook until fruit begins to release juice, about 10 minutes more. Add Sauternes wine, 2 cups cold water, and salt to skillet. Mix well. Bring mixture to a boil, reduce heat to medium low; let simmer until fruit begins to break down and liquid is very flavorful, 10 to 12 minutes. Remove from heat. Puree mixture through a food mill set over a bowl; discard solids. Pass puree through a fine mesh sieve set over a bowl. Discard solids.
Slice remaining plum into thin slivers, and add to soup. Add another third of the blackberries and another third of the raspberries to soup. Transfer to refrigerator until chilled.
Heat oven to 350 degrees. Slice mini brioche in half (if using loaf, slice into 1/2-inch-thick slices). Place brioche on a baking sheet; toast until golden, 10 to 12 minutes. Remove from oven, and set aside.
Combine mascarpone and heavy cream in a small mixing bowl. Fold together by hand. Spoon mixture over toasted brioche. Arrange remaining third of berries over mascarpone, and place two toasts into each bowl. Pour chilled soup in bowls, and serve.