Preheat the oven to 180°C(350°F). Grease and prepare a 2lb loaf pan 23 x 14cm(9x5.5inch) with parchment paper allowing the paper to hang over the sides.
Add the flour, sugar, oats, baking powder and salt into a medium mixing bowl and stir to combine. Add the butter and using your fingertips rub the butter into the flour until it resembles dried bread crumbs. Add the yolk and mix it in with your fingertips. You should be able to squeeze the mixture together and it forms into a light ball.
Pour ⅔ of the crumble mixture into the loaf pan, spread it out into an even layer and press down with your fingertips until it forms a flat base.
In a separate bowl, add the strawberries, rhubarb, lemon juice, sugar and cornflour and stir to combine. Pour the fruit filling on top of the crumble base in the loaf pan and spread out evenly.
Now layer the remaining crumble over the top of the fruit, allowing some of the fruit to show through. Bake in the oven for 40-45 minutes until golden brown on top.
Remove from the oven and allow to cool in the loaf tin. Once cooled, lift the crumble bar out of the pan using the overhanging sides of the baking paper and slice. (If you prefer handheld bars, then refrigerate the bars for 1-2 hours until set before slicing)