In a medium bowl, whisk the flour, baking powder, and salt.
Stir in both sugars until evenly distributed.
Drizzle in the cooled, melted butter while lightly tossing the mixture with a fork just until clumps form (pea-to-chickpea size). Do not mash smooth.
Refrigerate 10–15 minutes to firm up while you prep the peaches.
Make the filling:
Toss peaches & rest: In a large bowl, toss peaches with granulated sugar, lemon juice, and salt. Rest 10 minutes to draw out juices.
Add to baking dish: Spread peaches into a lightly buttered, 9 by 13-inch baking dish.
Top & bake: Scatter the chilled crumbs over the fruit, keeping them craggy with small gaps for bubbling. Sprinkle with turbinado sugar if using. Bake 40–45 minutes, until the topping is deep golden and the juices bubble thickly in the gaps. For extra crunch, broil 30–60 seconds—watch closely.
Rest & serve: Let stand 20–30 minutes so the sauce thickens. Serve warm with vanilla ice cream or whipped cream.
Notes
Butter temp matters: Melt, then cool until opaque/room-temp before mixing; warm butter makes the topping pasty.
Crumbs, not paste: Stop mixing as soon as you get clumps. If ever too wet, fork in 1–2Tbsp (8–16 g) extra flour and chill briefly.
Frozen peaches: Use the same 1–1.3 kg; thaw and drain off icy liquid first.
Optional thickener: If your peaches are extremely juicy or thawed from frozen, whisk cornstarch into the lemon juice after the 10-minute rest, then fold into the peaches before baking. Use about 1 Tbsp for firm-ripe fruit or 2 Tbsp for very ripe/juicy or thawed peaches.