How to Make It
Light a grill. In a medium saucepan, combine the sliced plums with 3/4 cup of the sugar and the water and bring to a boil. Cover and simmer over moderately low heat until the plums are very soft, about 10 minutes. Transfer the plums to a food processor and puree until smooth. Scrape the plum sauce into a bowl, stir in the lemon juice and set aside.
In a small bowl, stir the melted butter with the remaining 1/4 cup of sugar. Grill the halved plums and peaches over moderate heat, turning once, until the fruit is tender, about 6 minutes. Baste the plums and peaches with the butter and continue to grill, turning once and basting again, until caramelized and slightly charred, about 2 minutes longer.
Transfer the grilled fruit to plates and spoon the plum sauce on top. Dollop the mascarpone alongside the fruit and serve.