At least 8 hours before serving the tiramisu, combine
blueberries, granulated sugar, and lemon juice in a heavy-bottomed,
nonreactive medium saucepan. Bring to a boil over medium heat,
stirring occasionally to dissolve the sugar. Reduce the heat to
medium and simmer, uncovered, until the berries give off their
juices, about 5 minutes. Add the raspberries and cook until they
are heated through but still hold their shape, about 2 minutes.
Remove from the heat.
Arrange half of the ladyfingers in a 9 x 13-inch
trimming them as necessary to fit. Spoon half of the hot berry sauce
evenly over the ladyfingers. Top with the remaining ladyfingers,
then the remaining sauce. Let stand until cooled, about 30 minutes.
Combine the mascarpone and confectioner's sugar
in a medium bowl.
Using an electric mixer on low speed, gradually beat in the heavy
cream. Spread the mascarpone mixture over the ladyfingers. Cover
loosely with plastic wrap and refrigerate until chilled, at least
4 hours. Serve chilled.