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Preheat oven to 425F.
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Toss peaches with sugar, lemon juice, and cornstarch in a 2 quart baking dish and bake in middle of oven 10 minutes.
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While the peaches bake, stir together flour, sugar, baking powder, and salt. Blend in butter with your fingertips or a pastry blender until mixture resembles coarse meal. Stir in water until just combined.
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Remove peaches from oven and drop spoonfuls of topping over them. Bake in middle of oven until topping is golden, about 25 minutes. (Topping will spread as it bakes.)