Heat the oven to 375°F and butter a 9-by-13-inch baking dish.
Toss the peaches with 1/2 cup sugar, cornstarch, lemon juice and cinnamon and spread them in the dish.
Whisk the flour, baking powder, salt and remaining 1/4 cup sugar, then cut in the cold butter until pea-size pieces remain.
Stir in the milk just until a rough dough forms and drop 10 uneven spoonfuls over the peaches, leaving gaps.
Bake for 45 to 50 minutes until the biscuits are golden and the fruit bubbles vigorously in the center.
Cool for 25 minutes before serving so the peach juices thicken.