In a small sauce pan combine the sugar and water and bring to a boil. Swirl the pan until the sugar has dissolved completely. Pour the simple syrup into a small heatproof container, and let cool in the refrigerator. (If you're in a hurry, put it in the freezer to cool.)
In the bowl of a food processor, pulse the mascarpone with 1/3 cup of the simple syrup. Taste for sweetness and add a little more syrup if you like. Add the lemon juice to the mascarpone and pulse to combine; don't overwork it or it will curdle. Adjust the simple syrup and lemon juice to taste. Refrigerate the soured cream
In a bowl, toss the peach slices with 3 tablespoons of the simple syrup. Tear the mint leaves with your hands, add to the peaches and toss. Taste the peaches and add a little more syrup if you like. Let the peaches macerate in the syrup for 10 minutes.