Pulse flour, salt, and thyme, if desired, in a food processor until just combined. Add butter. Process until mixture resembles coarse meal, about 10 seconds. (To mix by hand, combine dry ingredients in a large mixing bowl. Using a pastry blender or 2 table knives, cut in butter until mixture resembles coarse meal.)

A Add 1/3 cup ice water, and process just until dough comes together, no more than 30 seconds (dough should not be sticky). Squeeze a small amount between your fingers; if dough is crumbly, add the remaining water, a little bit at a time.

Turn dough out onto a piece of plastic wrap; flatten into a disk. Wrap in the plastic, and refrigerate at least 1 hour (or overnight) before using.