Preheat your oven to 350°F. Grease a 10.5 x 7.5-inches baking dish.
In a large bowl, combine the rhubarb, strawberries, granulated sugar, light brown sugar, cornstarch, zest of the lemon, and vanilla extract. Mix well to coat the fruit evenly. Transfer the mixture to the prepared baking dish and spread it out evenly.
4 cups rhubarb, 4 cups strawberries, 3/4 cup granulated sugar, 4 tablespoons cornstarch, 1 teaspoon vanilla extract, zest of half a lemon
In another bowl, combine the rolled oats, all-purpose flour, light brown sugar, granulated sugar, ground cinnamon, and salt. Mix well.
1 cup old-fashioned rolled oats, 1 cup all-purpose flour, 3/4 cup light brown sugar, 1/4 cup granulated sugar, 1 teaspoon ground cinnamon, 1 teaspoon salt
Add the cold, cubed butter to the oat mixture. Using your fingers or a pastry cutter, work the butter into the mixture until it resembles coarse crumbs.
1/2 cup unsalted butter
Sprinkle the crisp topping evenly over the fruit filling.
Bake in the preheated oven for 40-50minutes, or until the topping is golden brown and the filling is bubbling.
Remove the crisp from the oven and let it cool for about 10 minutes. Serve warm with a scoop of vanilla ice cream or a dollop of whipped cream, if desired.