Place strawberries and rhubarb in a large bowl. Add sugar, corn starch, vanilla and lemon juice; stir to combine. Transfer to an 8x8 or 9x9 baking dish.
Mix oats, flour, and brown sugar together in a small bowl. Add butter and mix to combine until mixture is crumbly. Spread over fruit mixture.
Bake in preheated oven for 1 hour, until fruit mixture is thick and the topping is golden brown.