Preheat the oven to 425°. Butter a large glass or ceramic baking dish. In a bowl, toss the peaches with the butter and sugar. Arrange the peaches in the dish, cut sides down, and bake in the upper third of the oven for 15 minutes, turning once, until just tender.
Meanwhile, in a saucepan, bring the raspberries, preserves and 2 tablespoons of water to a boil. Strain the glaze into a small bowl.
Spoon the glaze over the peaches and bake for 15 minutes longer, turning once and basting, until tender; transfer to bowls and top with a dollop of mascarpone. Spoon the juices on top, garnish with amaretti and serve.