Mix flour, salt, and flaxseed in a food processor.
Add cold butter and cut in using a pastry blender, or pulse in the food processor.
Mix water, egg, and vinegar in a measuring cup.
Add 2 to 3 tablespoons ice water mix at a time, until dough forms into a ball. Gather up and pat into a disc. If possible, cover and refrigerate dough for 30 minutes before rolling out.
When ready to use, roll dough out on a lightly floured surface.