Preheat your oven to 350°F (175°C).
In a large mixing bowl, combine the sliced strawberries, chopped rhubarb, granulated sugar, cornstarch, vanilla extract, and lemon juice. Gently mix until the fruits are well coated. Pour this mixture into the 9x13 inch baking dish and spread it evenly.
In another bowl, combine the rolled oats, flour, brown sugar, ground cinnamon, and a pinch of salt. Pour in the melted butter and mix until the mixture is crumbly.
Evenly distribute the oat mixture over the fruit filling in the baking dish.
Bake in the preheated oven for 40 minutes, or until the topping is golden brown and the fruit is bubbling.
Remove from the oven and allow to cool for at least 10 minutes before serving. This will help the juices thicken slightly.
Serve warm, preferably with a scoop of vanilla ice cream or whipped cream.
If you have leftovers, store them in the refrigerator for up to 3 days. Reheat in the oven for a few minutes before serving.