Rhubarb seems to divide folks along lines of love and hate, but I often suspect the haters may have once been traumatized by a bowlful of overcooked rhubarb mush. Yet even when perfectly cooked, rhubarb's high levels of oxalic acid can sometimes create a tooth-coating chalkiness that would leave a bad taste in anyone's mouth. Oxalic acid is the same culprit behind the dreaded sensation of "spinach teeth" after a salad, and it exists at toxic levels in rhubarb leaves, so be sure to toss those out! The high but tolerable level in the rhubarb itself leads some bakers to bury their rhubarb in sugar, which makes for a painfully sweet/tart concoction.
With all those pitfalls, it's no wonder some folks are on the fence about (or outright hostile to) rhubarb, but happily those problems can all be solved with science and, perhaps, a dash of artistry when it comes to seasoning. And for those of us who already love rhubarb? These tricks will take that passion to the next level.
The first step is to make the streusel topping, which is what makes a crisp a crisp. Compared with a pie dough, streusels are much more forgiving, making it easy to swap one recipe for another. So if you've got an old family favorite, feel free to use that instead. While there's a "master formula" in my cookbook for making and customizing streusel for any recipe, this is the specific combo I like to pair with rhubarb.
It's a mix of brown sugar, whole wheat flour, and rolled oats, with cinnamon, anise seed, and Chinese five-spice powder. Plus butter—lots of butter—to enrich the otherwise lean fruit filling. Whole wheat flour gives the topping an extra-crispy sort of texture, combining with the brown sugar and cinnamon to create a graham cracker–like flavor, but if you don't have any on hand, all-purpose will do. If you're gluten-free, just grab your favorite APF-style blend.
The aroma of anise seed (along with the echoes of fennel and star anise in the five-spice powder) works wonders with rhubarb, coaxing out more of its naturally herbaceous flavor without overpowering its delicate taste—an idea I picked up from one of my favorite books, The Flavor Thesaurus.
Like adding a pinch of nutmeg to béchamel, a bit of ground cloves to banana bread, or a dash of coriander to my blueberry pie, using this spice combo isn't about making the spices stand out; it's about underscoring the main attraction, somehow making the rhubarb seem rhubarb-ier. If you like, the streusel can be made in advance—it'll keep for about a week in the fridge or a few months in the freezer.
If you've read up on my game plan for making the ultimate cherry pie, my formula for thickening a rhubarb crisp looks about the same on paper, although the method itself differs in a big way (more on that in a bit). Start with the weight of the prepared rhubarb—in this case, 44 ounces—then add 5.5% tapioca starch and 25% sugar. (This math is all much easier if you work in metric units, by the way.) The numbers in my recipe are rounded for convenience, but the percentages are handy to know if you'd like to scale a batch up or down depending on your farmers market haul.
If you're not a purist, up to a third of the rhubarb can be replaced with strawberries. Leave the little ones whole, but cut larger berries in half, or in quarters if they're huge.
Now, here's where things differ from my method for pie. The unique format of a crisp allows me to bake the fruit in stages, instead of tossing everything together, topping it with streusel, and baking it all at once. I start by combining half of the rhubarb with the sugar and starch, plus two ounces of elderflower liqueur, such as St-Germain. If you don't have any on hand, you can use water or an extra two ounces of fruit (rhubarb is about 90% water), but the delicate floral aroma of elderflower goes a long way toward opening up the subtleties of rhubarb.
Transfer the mixture to a seven- by 11-inch baking dish. (A deep-dish nine-inch pie pan will work as well, but I prefer the rectangular shape to maximize surface area for the streusel later on.) Cover the dish tightly in foil, and bake it until juicy. This takes about 30 minutes in a 400°F (200°C) oven, although the time can vary quite a bit depending on the dimensions and material of your baking dish, so check on the fruit a little early if you're using a metal dish.
Once the rhubarb has given up its juices, pull it from the oven and stir in a quarter teaspoon of baking soda using a heat-resistant spatula. It's not enough to fully neutralize the rhubarb's acidity, but it certainly takes off the sharper edges—a particularly helpful trick when you're working with hothouse rhubarb, which can taste especially sour. Spinach teeth, begone!
Keep stirring until the fizz dies down to ensure no pockets of baking soda remain unactivated, then pour the syrupy rhubarb over the remaining fruit—whether it's 100% rhubarb or a mix with strawberries. Fold gently to combine, then return to the baking dish.
Baking in stages produces a variety of textures in the finished crisp, with creamy pieces of rhubarb melting into a gooey filling alongside big chunks of firmer fruit. Top it off with the prepared streusel, smashing each handful into a thin sheet.
It may look a little strange, but this keeps smaller crumbles of streusel from falling down between the pieces of fruit and sinking into the filling; the "tiles" stay perched on top, where they can bake up nice and crisp. When the whole thing's covered, return to the oven and bake until bubbling-hot even in the very middle, about 30 minutes more.
This ensures the tapioca starch can do its thang. If it's not bubbling-hot, the filling won't thicken as it should, so let that be your cue rather than any strict timetable. Again, the exact size, shape, depth, and material of your baking dish will play an enormous role in how long this process will take. Larger pans, or those made of metal, will require less time in the oven, while smaller or deeper pans may require quite a bit more. You can absolutely bake with whatever type of pan you have on hand; just let your eyes be your guide, rather than a clock.
Let the finished crisp cool at least 30 minutes before serving, as it will be soupy and dangerously hot at first. As it cools to a safer temperature, the crisp will thicken up nicely, reaching what I consider its ideal consistency at about 80°F (27°C), at which point it will be saucy and warm.