For the Crust: Position one oven rack at highest position or wherever your oven is hottest and another rack lower down. Preheat oven to 400°F/200°C. Line the bottom of your oven with a sheet of aluminum foil to catch drips.

Whisk the eggs, water and vinegar together in a 2 cup (480 ml) measuring cup with spout and refrigerate until needed.

To Make by Hand: Whisk together the flour, sugar, if using, xanthan gum and salt in a medium bowl to blend. Add the butter and cut in, using a pastry blender or two knives, until the butter is cut into assorted sized pieces ranging from approximately 1/8 inch to 1/2 inch (3mm to12 mm). Sprinkle wet mixture over the dry and toss with fingers or a fork until evenly moistened and the dough just holds together if squeezed.

To Make in a Stand Mixer: Put the flour, sugar, if using, xanthan gum and salt in the stand mixer bowl. Using flat paddle attachment, turn on low speed to blend dry ingredients. Add the butter and pulse on and off until the butter is cut into assorted sized pieces ranging from approximately 1/8 inch to 1/2 inch (3mm to12 mm). Sprinkle wet mixture over the dry and pulse on and off until dough is evenly moistened and the just holds together if squeezed.

To Make with a Food Processor: Put the flour, sugar, if using, xanthan gum and salt in the bowl of a food processor fitted with the metal blade attachment and pulse to combine. Add the butter and pulse on and off until the butter is cut into assorted sized pieces ranging from approximately 1/8 inch to 1/2 inch (3mm to12 mm). Sprinkle wet mixture over the dry and pulse on and off until dough is evenly moistened and the just holds together if squeezed.

To Finish for All Techniques: Empty dough onto work surface and use hands to bring together gently. Divide in half, form each portion into a ball and flatten. This dough may be rolled out right away either on a lightly floured surface or between two pieces of parchment paper. You can also make ahead and refrigerate for up to 2 days or freeze for up to 1 month, in which case protect it further by placing in a plastic zipper-top bag; defrost in the refrigerator overnight. Let the dough soften slightly at room temperature before rolling out.

For the Filling: Place about one-quarter of the blueberries (just eyeball it) in a mixing bowl with cornstarch. Use a potato masher to mash the berries to encourage some moisture to combine with the cornstarch. Add remaining berries, 1 cup (198 g) of sugar and the lemon juice. Fold together very well to mix. Allow to sit for 15 minutes, folding well once or twice.

Coat a 9 inch (23 cm) pie plate with nonstick spray. Roll out bottom crust on work surface lightly dusted with extra flour to a 13 inch (33 cm) diameter circle and about 1/8 inch (3 mm) thick; gently fit into pie plate.

Pile fruit mixture into pie crust. Dot with butter. Refrigerate while rolling out top crust.

Roll out top crust on lightly flour dusted surface to about a 12 inch (30.5 cm) diameter and 1/8 inch (3 mm) thick and place on top of berries. Seal edges of both crusts together and trim away any excess, leaving about a 1 inch (2.5 cm) overhang. Fold both edges under and crimp. Use a sharp paring knife to make steam vents and a center hole in the top crust. Brush top of pie with milk and sprinkle with extra sugar if you like. Place pie in oven on hottest rack and bake for 30 minutes. Check to see how pie is browning, rotating pie from front to back for even baking. Cover loosely with aluminum foil at any point to prevent over-browning. Continue to bake for approximately 20 to 30 minutes more or until crust is golden brown and filling is bubbling. Cool pie on wire rack for at least 30 minutes before serving to allow juices to thicken.

Serve warm or at room temperature, with or without lactose-free vanilla ice cream or a dollop of lactose-free yogurt (for breakfast!). Pie may be stored at room temperature overnight lightly covered with foil.