Note: I used to make dough in a food processor. It was easy, to be sure. Feel free to do it that way if you are tight on time. But now, I make the extra time to cut the butter into the flour by hand. It’s not much longer really, and for the results, it’s worth it.
In a large bowl, stir together the dry ingredients. Add the butter and use a pastry blender to cut it into the flour, scraping its edge with a knife occasionally to free any clumps that form. Keep at it until you are left with pea-sized bits of butter in the mix, being careful not to overwork the dough. A few larger butter clumps are okay. Add the egg and 1–2 tablespoons of buttermilk and cut them in. As you continue to work them, the dough will form. Pinch the dough with your thumb and index finger to see if it holds together. If it’s crumbly, it isn’t ready. Drizzle another tablespoon of buttermilk (or more, as needed) and cut in again. Once the dough has formed, empty it out onto cellophane. Press opposite ends of the cellophane together to form the dough into a ball. Flatten it into a disk, wrap it securely, then refrigerate for at least 20 minutes to let dough mellow. In a bowl, mix together ricotta and Parmesan and season with salt. Set aside. Between sheets of lightly floured parchment, roll dough out to 14” round, about 1⁄8”–1⁄4” thick. Transfer to a rimmed baking sheet and peel top parchment layer away. Spread ricotta-Parm mixture onto pastry, leaving a 2” border all around. Layer tomato slices in a pleasing fashion. When I made this, it struck me as a stained glass window. Season with salt and pepper, then fold pastry over filling, pleating as you go. Brush buttermilk over pastry border, then refrigerate for at least 30 minutes to set. Preheat oven to 400oF. Remove gallette from fridge and bake in oven for 25–35 minutes, or until crust is deeply golden. Allow to cool for 5–10 minutes on a wire rack. Serve warm or at room temperature.