Prepare the fruit by chopping the rhubarb stalks into 2in/4cm pieces and slicing the strawberries in half.
In a large pan set over medium heat, add the fruit, sugar, vanilla and lemon juice.
Bring to a simmer then taste and adjust as necessary.
Because the strawberries and rhubarb can both be quite tart, you might need to add a little more sugar to balance the tartness. cook for 7-10 minutes until the juices have thickened and the mixture is jammy.
Transfer the crumble filling to a baking dish.
To make the crumble topping, rub cold, cubed butter into the flour and sugar in a large bowl with a pinch of salt until a rough, crumbly texture forms.
Scatter the crumble mixture over the fruit filling and place the dish in a foil-lined baking sheet (to catch any juices that might boil over).
Place in a preheated oven and bake for 20 minutes until the crumble is golden brown and crisp and the filling is bubbling.
Remove and serve warm with ice cold custard, cream or a scoop of vanilla ice cream.