Preheat oven to 425°F. Place a large baking sheet covered in foil on the lowest rack in the oven. This prevents overflow and catches drips.

In a large bowl, combine sliced peaches with lemon juice, sugars, cinnamon, nutmeg, and cornstarch. Mix well and pour into a 9x13 baking dish.

Bake the peach mixture for 10 minutes. Meanwhile, mix dry topping ingredients, toss grated butter into the flour mixture, then stir in boiling water until just combined. Drop spoonfuls over peaches.

Sprinkle the topping with cinnamon sugar. Bake until crust is golden, about 28 minutes. Serve warm or at room temperature.

Optional: Freeze the prepared filling in a bag for up to a year. Thaw and bake when ready.