Tips before getting started: The ice water must have ice in it. The butter must be cut into small pieces.
To make the dough: Stir the yogurt and the ice water (strained of ice) together in a small bowl and set aside.
Put the flour, cornmeal, sugar and salt in a large bowl, tossing them once or twice.
Work the butter into the flour mixture aiming for pieces that range in size from bread crumbs to small peas. The smaller pieces will make the dough tender, the larger ones will make it flaky.
Sprinkle the yogurt/water mixture over the dough, one tablespoon at a time.
With your hands, gather the curds of dough together. The dough will be soft. Do not overwork the dough. Divide the dough in half and shape into a disk. Wrap in plastic and refrigerate at least two hours or freeze up to one month.
Preheat oven to 400. Line a baking sheet with parchment. Put dough on a lightly floured work surface and roll in into an 8 - inch circle about 1/8th inch thick. If it's sticky, throw a little extra flour on it, your hands and the rolling pin. Transfer dough to baking sheet.
Top with any combination of sweet or savory ingredients. Leave a 2-3 inch border un-topped. Fold uncovered border of dough up over the filling, allowing the dough to pleat as you work your way around the galette. The dough will "pleat" and fall naturally. Let it.
If you're preparing a sweet or dessert galette, brush the crust with an egg wash and sprinkle with sugar just before baking.
Bake 35 to 40 minutes or until pastry is golden and fruits are bubbly. Transfer baking sheet to a cooling rack and let stand ten minutes or so. Slide galette on to a serving dish. Serve warm or room temperature.