Preheat oven to 425° F. Slice the tomatoes fairly thickly and lay out flat on paper towels. Sprinkle the tomatoes with kosher salt and let them sit for 15-20 minutes to draw out the water and concentrate the tomato flavor.
In a stand mixer with a paddle attachment combine the flour, baking powder, sugar, baking soda and salt. Blend until dry ingredients are combined. Now over the bowl of the stand mixer, cut the cold buter into small pieces and let them drop into the bowl. Blend the butter and dry ingredients until the mixture looks like pea-sized bits of butter.
Whisk together the sour cream and water so that it is the consistency of buttermilk (add more water if yogurt is really thick, but be careful not to get it too wet). Fold the sour cream mixture into the flour/butter mixture and form into a 5-inch round. The dough will be sticky.
Place some parchment paper on the countertop and sprinkle with cornmeal. Knead the 5” dough ball 6-8 times, place on the parchment paper and roll out with a rolling pin to a 13-inch round. Dust the rolling pin with flour when it starts to stick to the dough. When finished rolling out, sprinkle about a 9-inch circle of the dough with remaining cornmeal, leaving out about 1 1/2 inches around the outside edges. This is to keep the tomato filling from getting too soggy. Transfer the parchment paper with the dough onto a sheet pan.
Pat the tomatoes dry and arrange them in a concentric circle on the 9-inch portion of the dough, overlapping a bit if necessary. In a small bowl, combine cheese, mayonnaise and herbs. Dollop a bit of the cheese mixture on top of the tomatoes
With your hands, gently fold the dough up around the filling, forming “pleats” where it overlaps. It will be freeform and rustic. Do not fret. See the youtube video in the notes section for a detailed look. Wet your fingers with water and rub the dough to patch any holes. For a really pretty look, brush some whisked up egg on the dough and sprinkle lightly with kosher salt.
Bake 20-25 minutes until golden crust. Check to make sure cheese and crust are not getting too dark.