Preheat the oven: Heat the oven to 160°C for a fan oven or 170°C for a conventional oven.
Prepare the fruit: Cut the peaches into chunky wedges and place them in a large bowl. Add the fresh raspberries and caster sugar, then gently toss until the fruit is evenly coated.
Transfer the fruit: Tip the sweetened peach and raspberry mixture into an ovenproof baking dish and spread it into an even layer.
Make the topping: Add the plain flour, baking powder, and caster sugar to a large bowl. Rub in the cold butter with your fingertips until the mixture resembles coarse crumbs with some pea-sized pieces of butter remaining.
Add the milk: Add the milk and mix gently until the mixture forms a soft dough with a texture similar to soft cookie dough. Stir in the lemon zest.
Top the fruit: Spoon the cobbler topping over the peaches and raspberries, leaving some small gaps where the fruit can bubble through. Sprinkle with optional demerara sugar and the flaked almonds.
Bake: Place the peach and raspberry cobbler in the preheated oven and bake for 25–30 minutes, or until the fruit is juicy and bubbling around the edges and the topping is golden and fluffy.
Serve: Let the cobbler rest for a few minutes before serving. Enjoy it warm on its own or with your favorite accompaniment.