Why It Works
- Whole wheat flour gives the streusel a graham cracker–like flavor.
- Anise seed and Chinese five-spice powder complement rhubarb, coaxing out more of its natural aroma, while cinnamon adds earthy depth.
- Tapioca starch forms a light, clear gel that keeps the filling gooey and thick, never gloppy.
- The sweet and complex aroma of elderflower helps open up the floral notes in rhubarb.
- Baking in stages creates the ideal ratio of gooey filling and tender fruit.
- Baking soda mellows the sharp acidity of rhubarb, lessening its sometimes chalky mouthfeel.
This crisp is everything a crisp should be: tender pieces of rhubarb in a thick sauce under a crunchy layer of streusel. The trick is baking the rhubarb in stages, then stirring in a pinch of sodium bicarbonate to help neutralize some of its sharp acidity. The result is tart but mellow, and beautifully aromatic.
If you're not into the idea of pure rhubarb, try a Strawberry-Rhubarb Crisp instead.
- For the Topping:
- 5 ounces light brown sugar (about 2/3 cup, packed; 140g)
- 3 3/4 ounces whole wheat flour (about 3/4 cup, spooned; 105g)
- 4 ounces old-fashioned rolled oats (about 1 1/3 cups; 115g)
- 1 1/2 teaspoons ground cinnamon
- 1 teaspoon whole anise seed (optional)
- 1 teaspoon Chinese five-spice powder
- 1/4 teaspoon (1g) Diamond Crystal kosher salt; for table salt, use half as much by volume or use the same weight
- 5 ounces unsalted butter (10 tablespoons; 140g), soft but cool, about 68°F (20°C)
- 5 ounces raw pecan pieces (about 1 cup; 140g)
- For the Filling:
- 2 3/4 pounds rhubarb, cut into 3/4-inch pieces (about 11 cups; 1.25kg), from around eighteen 24-inch stalks (3 pounds; 1.35kg)
- 10 1/2 ounces sugar (about 1 1/2 cups; 300g)
- 2 1/2 ounces tapioca flour (about 2/3 cup; 75g), such as Bob’s Red Mill
- 2 ounces elderflower liqueur or water (1/4 cup; 55g)
- 3/4 teaspoon (3g) Diamond Crystal kosher salt; for table salt, use half as much by volume or use the same weight
- 1/4 teaspoon baking soda
- To Serve (optional):
- 1 recipe strawberry whipped cream or brown sugar whipped cream
For the Topping: Combine brown sugar, whole wheat flour, rolled oats, cinnamon, anise seed (if using), five-spice powder, salt, and butter in the bowl of a stand mixer fitted with a paddle attachment. Mix on low speed to form a thick dough, about 5 minutes. Alternatively, this can be done by hand. Toss in pecans, transfer to a zip-top bag, and refrigerate until needed, up to 1 month.
For the Filling: Adjust oven rack to lower-middle position and preheat to 400°F (200°C). In the same bowl from the stand mixer (no need to wash), combine half the prepared rhubarb (about 5 1/2 cups; 22 ounces; 620g), sugar, tapioca flour, elderflower liqueur or water, and salt with a flexible spatula; the mixture will be extremely dry. Transfer to a 7- by 11-inch baking dish or 9-inch deep-dish pie pan, sprinkling any remaining sugar/starch on top, and cover with foil. Place on a foil- or parchment-lined half-sheet pan and bake until rhubarb is wilted and juicy, about 30 minutes. Meanwhile, transfer remaining fruit to the same bowl.
To Finish: Remove baking dish from oven, discard foil, and sprinkle with baking soda. Stir carefully with a heat-resistant spatula until juices begin to fizz. Pour over remaining rhubarb and fold to combine, then return to baking dish. Top with handfuls of prepared streusel, flattening each addition into a thin sheet to achieve even coverage. Continue baking until crisp is bubbling in the very center, about 35 minutes more. (The time will vary dramatically depending on the exact dimensions and material of your baking dish.)
Cool at least 30 minutes before serving, as the filling will be dangerously hot and extremely runny at first. The crisp will thicken as it cools to room temperature. If you like, serve à la mode or with dollops of strawberry or brown sugar whipped cream. Wrapped in foil, the crisp will keep 3 days at room temperature.
Due to disparate sourcing practices, tapioca starch manufactured outside the US may be derived from sago, which has different gelling properties from true tapioca (cassava). Look for brands that mention cassava by name; my favorite is Bob's Red Mill, available in the baking aisle of most supermarkets or online.
Make-Ahead and Storage
Wrapped in foil, the crisp will keep 3 days at room temperature.