How to Make It
In a large bowl, using a handheld mixer, beat the egg yolks with 3/4 cup of the sugar at medium-high speed until fluffy, 3 minutes. In a saucepan, combine the milk with the remaining 2 tablespoons of the sugar and bring to a simmer. Slowly beat the warm milk into the egg yolks at low speed. Scrape the custard into the saucepan. Cook over moderate heat, stirring constantly with a wooden spoon, until thick enough to coat the back of the spoon, about 5 minutes; don't let the custard boil.
Pour the custard into a bowl set in a larger bowl of ice water and whisk in the mascarpone, lemon juice and salt. Let stand until chilled, stirring occasionally, 30 minutes.
Pour the custard into an ice cream maker and freeze according to the manufacturer's instructions. Transfer the mascarpone ice cream to an airtight container and freeze until firm, at least 2 hours.
In a large saucepan, combine the white wine, honey, water and sugar and bring to a boil. Boil until reduced by half, about 30 minutes. Add the rosemary sprig and let stand for 10 minutes; discard the rosemary.
Preheat the oven to 350°. Arrange the peaches in an 8-by-11-inch baking dish. Pour the rosemary syrup on top and roast the peaches until tender, 40 minutes, basting and turning the peaches occasionally.
Scoop the mascarpone ice cream into bowls and top with the peach halves. Spoon the warm poaching liquid over the fruit and serve right away.