Strawberries and rhubarb have been paired together for so long that we think of them as a great match. Except that they’re not. Both are loaded with water (92 and 95 percent water by weight, respectively). When the two are enclosed in pastry and baked, that water heats up and causes both to soften dramatically, albeit differently: Rhubarb often blows out completely, releasing all that moisture into the filling and collapsing into mush, while strawberries remain intact but become unappealingly bloated. Our task was to bring together delicately sweet strawberries and bracingly tart rhubarb in a bright-tasting filling that gels softly and contains plenty of intact fruit and vegetable pieces.