Preheat oven to 425°F; lightly grease a 9x13-inch dish.
Gently toss peaches, 3/4 cup granulated sugar, brown sugar, cinnamon, nutmeg, lemon juice, and cornstarch in a large bowl. Stand 10 minutes; pour into dish.
Mix flour, 1/4 cup sugar, baking powder, and salt in another bowl; cut in cold butter with pastry cutter or fork to coarse crumbs.
Stir in boiling water just until combined. Drop batter spoonfuls over peaches, leaving steam gaps.
Bake 40-45 minutes until topping is golden brown and fruit bubbles at edges.
Cool 15 minutes; serve warm with vanilla ice cream or whipped cream.