For the pastry: Whisk-stir the flour and salt in a large bowl. Sprinkle bits of the butter over the dough and, using a pastry blender or your fingertips, work it into the flour until the mixture resembles coarse meal, with the biggest pieces of butter the size of tiny peas. In a small bowl, stir together the yogurt or sour cream, lemon juice and about 1/4 cup water and add this to the butter-flour mixture. With your fingertips or a wooden spoon, mix in the liquid until large lumps form. Pat the lumps into a ball. Wrap with plastic and refrigerate for 1 hour, or up to 2 days.
For the filling: Get down the saute pan with the lid. If you don't have one, any large lid will do. Add the olive oil, tomatoes, salt and a pinch of red pepper flakes (if that's your thing) to your saute pan, then cover and heat over high heat. Roll the tomatoes around from time to time so that they'll cook evenly. In a few minutes, you'll hear some putts and pops as the tomatoes burst a little. When most have, remove the lid, turn the heat down to medium and add the zucchini chunks. Saute until they soften, about 2 minutes. Add the corn and cook 1 minute. Add the scallions, just stirring them in, then turn off the heat. Adjust the seasonings if needed. Transfer the mixture to a large plate and spread it out so that it will cool faster. (You want it cooled to at least lukewarm before assembling the galette.)
Assemble the galette: Preheat the oven to 400 degrees F.
On a floured counter, roll the dough out into a 12-inch round (it really doesn't need to be perfectly shaped). Transfer to a parchment-lined baking sheet; I like to fold my dough gently, without creasing, in quarters, then unfold it onto the baking sheet. Sprinkle the tomato-zucchini-corn mixture with half of the Parmesan and spoon the mixture into the center of the dough, leaving a 2-inch border. If any liquid has puddled in the plate, try to leave it there as you spoon. Sprinkle with almost all of the remaining Parmesan, leaving a pinch or two behind for the crust. Fold the border over the filling, pleating the edge to make it fit. The center will be open. Brush the crust with the egg yolk glaze. Sprinkle the glaze with the last pinches of Parmesan.
Bake until puffed and golden brown, 30 to 40 minutes. Remove from the oven and let stand for 5 minutes, then slide the galette onto a serving plate.