For the dough:
1 1/4 cups all-purpose flour
1/4 teaspoon table salt
8 tablespoons cold unsalted butter, cut into pieces and chill again
1/4 cup plain yogurt
2 teaspoons fresh lemon juice
1/4 cup ice water

Make dough: Whisk stir the flour and salt in a large bowl. Sprinkle bits of butter over 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, lemon juice, and 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.

Make filling: Add all ingredients except the peas and the cornstarch.  Cook until the potatoes are firm, but piercable with a fork.  Add the cornstarch to thicken.  Add the peas and cook for two minutes.

Assemble galette: Heat oven to 400 degrees. On a floured counter, roll the dough out into a 12-inch round and it really doesn’t need to be perfectly shaped. Transfer to a parchment-lined baking sheet. Sprinkle the filling into the center of the dough, leaving a 2-inch border. If any liquid has puddle in plate, try to leave it there as you spoon. Fold the border over the filling, pleating the edge to make it fit. The center will be open. Brush crust with egg yolk glaze. Sprinkle glaze with last pinches of parmesan.

Bake the galette: For 30 to 40 minutes, or until puffed and golden brown. Remove from the oven and let stand for 5 minutes, then slide the galette onto a serving plate. Cut into wedges and serve hot, warm or at room temperature.