Place the strawberries, raspberries, orange segments (plus any juice), sugar and cinnamon in a bowl and toss together gently to coat the fruit lightly in the sugar, taking care not to break up the raspberries. Transfer the fruit to a gratin dish and set aside while making the crumble topping.
Place the flour into a bowl and using your fingertips rub the butter into it until the mixture resembles fine breadcrumbs. Stir in the sugar and nuts, if using, then spoon the crumble over the top of the fruit. Bake in a preheated oven, 200°C (400°F), Gas Mark 6, for 25–30 minutes until the topping is golden and crisp in places.
Make the pink custard: place the egg yolks, cornflour and sugar in a bowl and blend together well. Put the milk in a heavy-based nonstick saucepan and bring to the boil. Pour the milk over the egg mixture, add the beetroot juice and whisk together well. Return to the heat and cook gently, stirring continuously until thickened.
Serve the crumble in bowls with the pink custard to spoon over.