Whisk whole-wheat flour, all-purpose flour, sugar and salt in a medium bowl. Cut in butter with a pastry blender or your fingers until the mixture resembles coarse crumbs with a few larger pieces. Add oil and stir with a fork to blend. Mix 1/4 cup water, egg yolk and lemon juice (or vinegar) in a measuring cup. Make a well in the center of the flour mixture. Add enough of the egg yolk mixture, stirring with a fork, until the dough clumps together. (Add a little water if the dough seems too dry.) Turn the dough out onto a lightly floured surface and knead several times. Form the dough into a ball, then flatten into a disk. Wrap in plastic wrap and refrigerate for at least 1 hour.