Begin by making the dough. Whisk flour, cornmeal, and salt together in a large bowl, until combined.
Fill a small bowl with ice-cold water. Add two teaspoons of apple cider vinegar to another small bowl. Set both aisde.
Add the cold, solid cubes of butter to the flour-cornmeal mixture. Using a pastry cutter, cut the butter into the dry ingredients until the butter is the size of small peas. The mixture should slightly resemble coarse meal.
Mix two tablespoons of the cold water with the apple cider vinegar. Sprinkle over the flour/butter mixture and begin to gently knead and shape the dough. Add small amounts of the ice water (about 1 teaspoon at a time) to the dough, until it comes together without crumbling. It should be moist and slightly sticky.
Shape the dough into a disc. Cover it with plastic wrap, and let chill in the refrigerator overnight.
Preheat the oven to 375 degrees Fahrenheit.
Add ricotta, swiss, and mozzarella cheese to a medium-sized bowl. Add chopped oregano, salt, pepper, and garlic powder. Cream the cheeses and the seasonings together, until smooth.
Remove the dough from the refrigerator and let it soften for about 6-10 minutes. Place the dough on a large sheet of parchment paper. Place another sheet of parchment paper over the dough. Lightly flour a rolling pin, and roll the dough out to a 12 inch circle, about 1/4 inch thick. If the dough feels warm after rolling, place it in the refrigerator for a little while to cool.
Transfer the parchment paper with the dough to a lightly greased sheet pan. Discard the other sheet of parchment paper.
Dollop the cheese mixture on top of the dough. Use a butter knife to spread it evenly, leaving about a 2-inch border at the edges. Then, take the edges of the dough and carefully fold it over the filling. In a small bowl, lightly whisk an egg. Brush the dough with the egg.
Sprinkle the galette with extra oregano and a very small amount of salt. Bake for about 20 minutes, then loosely cover the top of the pastry with aluminum foil, to prevent over-browning.
Bake for another 20-25 minutes, checking occasionally to make sure the cheese isn't burning or over-cooking.
Once finished, remove from the oven and let the galette cool slightly before slicing into it. Serve warm, as an appetizer or side dish.