Preheat your oven to 350°F (175°C).
In a large mixing bowl, combine the chopped rhubarb and halved strawberries. Add the granulated sugar, flour, vanilla extract, and lemon juice. Gently toss the mixture until the fruit is evenly coated. Pour the fruit mixture into the prepared 9x13 inch baking dish.
In another mixing bowl, combine the flour, rolled oats, brown sugar, cinnamon, and salt. Pour in the melted butter, and mix until crumbly and well combined.
Evenly spread the crisp topping over the fruit filling in the baking dish.
Bake in the preheated oven for about 40 minutes, or until the top is golden brown and the fruit is bubbly.
Remove from the oven and let it cool for a few minutes before serving.
Serve warm, with a scoop of vanilla ice cream or whipped cream for added indulgence.
Store leftovers in the refrigerator for up to 3 days and reheat in the oven before serving for the best texture.