Start this recipe by preparing the filling. Cut the rhubarb stalks into 2cm pieces. Place in a saucepan with the sugar, lemon zest and a few tablespoons water. Cook together for a few minutes until the rhubarb is cooked. Remove from the heat, add the strawberries and pour into a greased ovenproof pie dish. For the custard, Beat the egg yolks and sugar together in a bowl until well combined. Add the corn flour and mix well. Place the milk powder, water and vanilla pod to a saucepan and bring to the boil. Pour the hot liquid over the egg yolk mixture and whisk well. Pour back into the saucepan and continue to cook, stirring until the custard is thick enough to coat the back of a spoon. Remove off the heat and pour the custard over the cooked rhubarb and strawberries. Mix lightly to combine. Finally, prepare the crumble topping. Rub the flour, ground almonds, butter and sugar together until the mixture resembles breadcrumbs. Stir in the flaked almonds and carefully spoon the mixture over the strawberry and rhubarb filling. Bake in the oven at 200`c for 20-25 minutes or until the crumble is just beginning to brown. Serve either hot or cold.