The simple elegance of this rhubarb crisp recipe means that there aren't a lot of extra ingredients, like flour or cornstarch, to thicken up the juices in the filling. This means that you're relying on cooking down the fruit to get a good consistency. Don't overcook the fruit but do let the juice simmer enough to reduce. It should only take 5 minutes and you're going to want to keep an eye on it. Also, keep in mind that the filling will set up a little bit as it cools, so you might be just fine.