Combine sugar and cornflour in a large saucepan; stir in rhubarb and strawberries and bring to a simmer over medium heat. Cook until thickened, about two minutes. Remove from heat, stir in lemon juice and let cool.
Preheat oven to 180 C / Gas 4. Lightly grease a 9x17.5cm baking tin.
Combine 375g flour, 200g sugar, baking powder and bicarbonate of soda in a large bowl. Rub in butter until mixture resembles coarse sand. Beat together eggs, buttermilk and vanilla in a separate bowl. Stir egg mixture into flour until just moistened. Spoon two-thirds of the mixture into prepared cake tin; evenly spread on the cooled filling, then cover with remaining mixture.
For the topping: combine sugar and flour; rub in butter until the mixture resembles coarse crumbs; sprinkle over the top of cake. Bake in preheated oven for 45 to 50 minutes or until golden brown. Cool before serving.
Made this for an elderly neighbour on return home from hospital, he didn't get it all! Used my own buttermilk mix (milk and tsp of lemon juice) but other than that followed the recipe, I did mix my own museli in the top layer for some crunch. - 07 Apr 2016