Preheat oven to 375°F (190°C). Grease a 9x13-inch baking dish.
Prepare the fruit filling: In a large bowl, combine rhubarb, strawberries, sugar, cornstarch, vanilla extract, and lemon zest. Let sit for 15 minutes, then drain any excess liquid.
Make the crisp topping: In another bowl, mix oats, flour, brown sugar, cinnamon, and salt. Cut in the cold butter until the mixture resembles coarse crumbs.
Assemble: Spread the fruit filling evenly in the prepared dish. Sprinkle the crisp topping evenly over the fruit.
Bake: Place in the oven and bake for 40-45 minutes, until the topping is golden brown and the fruit is bubbling.
Cool & Serve: Let it cool for at least 10 minutes before serving. Enjoy warm with ice cream or whipped cream.
Notes
For a healthier version: Swap brown sugar for coconut sugar and use coconut oil instead of butter.
Gluten-free option: Replace all-purpose flour with almond flour or a gluten-free flour blend.
To prevent a runny filling: Increase cornstarch by 1 tablespoon.
For extra crunch: Add chopped pecans or walnuts to the topping.