Ripe fresh berries are naturally the best choice for this pure fruit fondue. Out of season, frozen berries can still make a memorable sweet tart dessert, but be sure to choose unsweetened, individual quick frozen berries. For dipping, offer shortbread or crisp gingerbread cookies, bite size cubes of pound cake, chocolate or hazelnut biscotti, apple or nectarine wedges, or chilled cubes of mild cheese, such as Brie or Havarti.
Puree strawberries and raspberries in food processor til smooth, working in batches if needed. Strain puree through a sieve to remove the seeds, pressing and working the bits in sieve to extract as much of the fruit and juice as possible.
Put berry puree in medium nonreactive pan. Stir in apple juice and sugar
In small bowl, whisk the cornstarch with liqueur til completely blended. Whisk into berry puree and bring to boil over medium heat, stirring. Reduce heat to low and simmer, stirring frequently til puree is slightly thickened, about 3 minutes. Add blueberries and cook til just heated through, about 3 minutes. Do not let the berries burst
Transfer to ceramic fondue pot set over a low flame
Makes about 6 to 8 servings, or 2 3/4 cups