Spread 1 1/2 tablespoons dulce de leche evenly on bottom of each gratin dish, then divide blueberries evenly among dishes. Arrange 4 plum quarters in each dish.
Put gratin dishes on a baking sheet and broil 4 to 6 inches from heat until dulce de leche begins to bubble, about 4 minutes, then sprinkle evenly with nuts and broil until nuts are browned, 1 to 2 minutes more. Serve warm.