Prepare the peaches: Bring a large pan of water to a rolling boil. Score a small cross on the bottom of each peach, then lower them into the water for 30–40 seconds. Use a slotted spoon to transfer them to a bowl of iced water. The skins will slip off easily. Slice each peach into 1 cm wedges, discarding the stones. You should have about 800 g of prepared fruit.

Cook the filling: In a large frying pan or skillet, combine the peach slices, caster sugar, cornflour, cinnamon, nutmeg, and lemon juice. Place over a medium heat and stir gently with a wooden spoon. After 2–3 minutes, you will see the sugar dissolve and the juices begin to pool around the fruit. The mixture will turn a deep golden amber as it thickens. Stir in the vanilla extract, then remove from the heat. The filling should be glossy and fragrant, with the peaches just softened at the edges.

Preheat the oven: Set your oven to 190°C (170°C fan) or Gas Mark 5. While it heats, make the topping.

Make the biscuit topping: In a large mixing bowl, whisk together the plain flour, baking powder, and salt. Add the cold, cubed butter. Using your fingertips, rub the butter into the flour until the mixture resembles coarse breadcrumbs — you should still see small, pea-sized flecks of butter. Pour in the milk and stir with a fork until a soft, slightly shaggy dough forms. Do not overwork it; the dough should be just combined and a little sticky.

Assemble the cobbler: Pour the warm peach filling into a 23 cm x 33 cm baking dish (or similar 2-litre capacity dish). Drop spoonfuls of the biscuit dough over the top — aim for about 8 rough mounds, leaving small gaps between them so the filling can bubble through. Brush the dough with the beaten egg, then sprinkle the demerara sugar evenly over the top.

Bake: Place the dish on a baking tray to catch any drips. Bake for 40–45 minutes, rotating halfway through. The cobbler is ready when the filling is bubbling vigorously around the edges and the topping is puffed, deep golden brown, and firm to a light touch. You should hear a gentle hiss as the juices bubble against the dish.

Rest before serving: Remove from the oven and let it sit for at least 15 minutes. This rest time allows the filling to thicken further and the topping to settle. Serve warm, not piping hot, to enjoy the full texture.