Peel and pit 1.5kg of ripe peaches, then cut into wedges. Toss with ¼ cup of superfine sugar and leave for 40 minutes.
Meanwhile, preheat the oven to 410°F / 210°C. Drain the peaches, reserving the juice. Mix ¼ cup of reserved peach juice with 1 teaspoon of cornstarch, 1 tablespoon of lemon juice, and ⅛ teaspoon of salt. Toss with the peaches.
Spread in an 11 × 7in / 28 × 18cm glass or ceramic baking dish. Bake for 12 minutes.
Mix 1¼ cups of all-purpose flour, ⅓ cup of superfine sugar, 1 teaspoon of baking powder, ½ teaspoon of baking soda, and ¼ teaspoon of salt. Rub in 85g of cold unsalted butter until crumbly.
Gently fold in ½ cup of plain full-fat yogurt. Drop clumps over the peaches, leaving gaps.
Sprinkle with 1 teaspoon of demerara sugar and ½ teaspoon of cinnamon. Bake for about 20 minutes, until golden and the topping reaches 203°F / 95°C inside.
