In a food processor, combine the flour (or flours), salt, and sugar. Pulse once to sift. Scatter half the butter over the flour. Pulse 5 or 6 times. Repeat with remaining butter and pulse until the mixture resembles coarse oatmeal. Add ⅓ cup cold water or stock and the verjus or vinegar and pulse once or twice. Do not process to a ball. Turn out onto a lightly floured work