Preheat your oven to 375°F (190°C). Grease a 9x13-inch baking dish or a similar-sized oven-safe dish.
In a large mixing bowl, combine the chopped rhubarb, sliced strawberries, granulated sugar, flour, vanilla extract, and lemon zest. Toss until the fruit is evenly coated with the sugar and flour mixture.
Transfer the fruit mixture to the prepared baking dish, spreading it out evenly.
In another mixing bowl, combine the rolled oats, flour, brown sugar, cinnamon, and salt for the crisp topping. Add the cold cubed butter and use your fingers or a pastry cutter to mix until the mixture resembles coarse crumbs.
Sprinkle the crisp topping evenly over the fruit mixture in the baking dish.
Bake in the preheated oven for 35-40 minutes, or until the topping is golden brown and the fruit is bubbly.
Remove from the oven and let it cool for a few minutes before serving.
Serve warm, preferably with a scoop of vanilla ice cream or a dollop of whipped cream.