1) Preheat the oven to 180°C.
2) Immerse the peaches in boiling water for 30 seconds to 1 minute, until their skins peel off easily. Place them immediately in cold water.
3) Peel the peaches, slice them into thick wedges, and place them in a large bowl. Add the lemon zest, lemon juice, caster sugar, and flour. Toss well. Gently mix in the blueberries. Allow the mixture to sit for 5 minutes. Spoon the mixture into ramekins or custard cups.
4) For the topping, combine the flour, caster sugar, brown sugar, salt, cinnamon, and the butter in the bowl of an electric mixer fitted with a paddle attachment. Mix on low speed until the butter is the size of peas. Rub the mixture with your fingertips until it's in big crumbles, then sprinkle evenly over the fruit.
5) Place the ramekins on a baking tray lined with baking paper and bake for 40 to 45 minutes, until the tops are browned and crisp and the juices are bubbly.
6) Serve warm or at room temperature.
If you want to make these early, store the unbaked crumbles in the refrigerator and bake before dinner.