Toss the rhubarb, strawberries, granulated sugar, cornstarch, lemon juice, vanilla, and half the salt in a large bowl. Let the fruit sit for 10 minutes while you preheat the oven to 375°F (190°C).

Stir together the oats, flour, brown sugar, cinnamon, and remaining salt in another bowl. Work the cold butter into the dry mix with your fingertips until it is crumbly with some pea-sized butter pieces, about 2 to 3 minutes.

Scrape the fruit and juices into a 9-inch square baking dish. Squeeze handfuls of the topping, break them into craggy pieces over the fruit, and cover it without packing down.

Place the dish on a baking sheet and bake for 35 to 40 minutes, until the topping is golden and the fruit bubbles thickly around the edges.

Rest the crisp on a wire rack for 10 minutes, then spoon it into bowls and serve warm.