Perfect pie dough has just the right balance of tenderness and structure. The former comes from fat, the latter from the long protein chains, called gluten, that form when flour mixes with water. Too little gluten and the dough won’t stick together; too much and the crust turns tough. So presumably we would face mostly a structural issue with a gluten-free dough, since gluten-free flours are naturally low in protein.
As our first step, we swapped in our gluten-free flour blend for the wheat flour in all the pie dough recipes the test kitchen has developed over the years. We produced workable doughs in every case, but an all-butter dough (which includes sour cream for tenderness) had the necessary richness to stand up to the starchiness of the gluten-free flour blend and was clearly the best starting point.
Although we weren’t surprised to find that the dough was still too soft and lacked structure, we were taken aback by how tough it was; on its own, the sour cream was not sufficient to tenderize a gluten-free dough. We solved the structural problem easily with the addition of a modest amount of xanthan gum, but flakiness and tenderness were still elusive. In an effort to further tenderize our dough, we tested ingredients that are known to tenderize: baking soda, lemon juice, and vinegar. Vinegar was the clear winner, producing a pie crust that was not only tender, but also light and flaky.
With the dough in a good place, we focused next on the blueberry filling. To thicken the pie, we tried cornstarch as well as our gluten-free flour blend but preferred tapioca starch, which is subtle enough to allow the berry flavor to shine through. Too much of it, though, created a congealed mess. Cooking some of the blueberries down to a thick saucy consistency helped us reduce the amount of tapioca required, as did adding a peeled Granny Smith apple that we shredded on the large holes of a box grater. Rich in pectin, the apple helped thicken the berries and enhanced their flavor.
And since gluten-free pie crusts can easily turn soggy, we found that preheating a sheet pan in the oven and baking the pie on the lower rack helped keep the crust crisp. It is important to use a metal pie plate in this recipe; it is not safe to place a glass (Pyrex) pie plate on a preheated baking sheet. If you must use a glass pie plate, do not preheat the baking sheet; note, however, that your crust will not be as crisp.