Preheat your oven to 350°F (175°C).
In a large mixing bowl, combine the chopped rhubarb, halved strawberries, granulated sugar, ¼ cup flour, and vanilla extract. Mix well and let the fruit mixture sit for about 10 minutes.
Pour the fruit mixture into the bottom of a 9x13-inch baking dish, spreading it evenly.
In another bowl, mix together the rolled oats, brown sugar, ½ cup flour, ground cinnamon, and a pinch of salt.
Pour the melted butter over the oat mixture and stir until everything is well combined and crumbly.
Evenly sprinkle the oat mixture over the top of the fruit layer in the baking dish.
Bake in the preheated oven for 45 minutes, or until the fruit is bubbling and the topping is golden brown.
Remove from the oven and allow to cool for a few minutes before serving.
Serve warm with a scoop of ice cream or whipped cream for a delicious treat.