Soak apricots in cold water over night and drain. Heat sugar and water together over medium heat for 10 minutes, then add apricots. Cook until aprocots are tender and a syrup forms. Add lemon juice, cook 1 minute more, and remove from heat. With a slotted spoon, transfer apricots to a plate to cool. With a spoon, open the apricots halfway and fill the inside with a small dollop of marscapone. Do not overstuff or the apricots will leak! Arrange the apricots, slit side up, on a platter and pour as much syrup as they will absorb over them. Garnish with the pistachio nuts.