Preheat oven to 180°C/Gas 4.
Mix the rhubarb and the strawberries together with 2 tablespoons of the sugar and transfer them to a snug fitting baking dish.
Put the flour, butter and remaining sugar in a bowl. Bring it together and work it with your fingertips until it resembles coarse breadcrumbs. Alternatively, pulse it together in a food processor to get the same result. Stir in the oats and a pinch of salt. Crumble it evenly over the top of the fruit. Wet your hands and flick water across the top of the crumble – this stops it browning too fast before the fruit has cooked.
Transfer to the oven and bake for 30-40 minutes, until golden on the top and bubbling beneath. Leave to cool for 10 minutes before serving, ideally with clotted cream.
