Combine the oats, cornmeal, sugar, and salt in a food processor. Pulse to make a gritty meal. Add the flour and pulse to mix. Scatter the butter over the dry mixture and pulse five or six times, until the butter is cut into pieces the size of split peas. Combine the water and egg yolk in a small bowl and blend with a fork. Sprinkle all of the liquid over the flour mixture in the processor. Pulse just until the mixture starts to form clumpy crumbs. Do not overprocess. Dump the crumbs onto your counter or into a large bowl. Pack the dough together, as you would a snowball, but don’t overdo it. Place it on a piece of plastic wrap. Flatten into a ¾-inch-thick disk. Wrap tightly in the plastic and refrigerate for 1 to 1½ hours before rolling. The dough should be slightly pliable, not rock-hard, when you roll it.