The pastry dough can be made up to 2 days in advance, or up to right after the beef goes into the oven—just be sure to give it an hour in the fridge to chill.
Put the flour, salt, sugar and thyme leaves in a food processor and pulse until the thyme is finely minced.
Drop in the butter and pulse about 10 times, until the mix resembles wet sand and most of the butter is cut down to the size of peas (though don’t worry if some pieces are larger).
Mix the water and vinegar in a small liquid measuring cup and, with the processor running now, pour in the water. You want it to be just moist enough to hold together.
Turn it out onto a lightly floured surface, form it into a ball, roll into a disc, and cover tightly with plastic wrap before stashing in the fridge.