Preheat the oven to 350F (176C).
Wash and prep the rhubarb and strawberries by cutting the rhubarb into 1/2 inch pieces, removing the leaves from the strawberries and halving/quartering them (depending on the size!).
Add the fruit to a bowl and mix in sugar, vanilla extract, a splash of lemon juice, and a pinch of salt. Add cornstarch if you would like and mix together well - set aside while you prepare the topping.
Add flour, oats, brown sugar, and salt to a bowl and stir together to combine.
Cube the cold stick of butter and add to the bowl. Crumble between your fingers until the butter has broken down into smaller pieces (largest chunks should be pea sized) and the mixture resembles coarse sand (with oats).
Add your fruit mixture to a pan - we used a 9 inch pie pan, but you can use a rectangular or square pan instead!
Spread the topping over the fruit evenly.
Bake for 45-55 minutes, until the fruit has cooked down and released its liquid - it should be bubbling around the sides! The topping should have a golden color to it as well. You can leave it in the oven for longer if needed!
Let cool for around 15 minutes, then serve warm - we love to top ours with ice cream or whipped cream! Enjoy!