Step 1In a food processor, combine the flour and salt and pulse. Mix the oil and water together, and with the machine running, add the oil and water mixture and process to make a smooth, soft dough, about 30 seconds. Add more flour or water if necessary, until the dough pulls off the sides of the food processor and forms a ball around the blade. The dough should be soft and slightly sticky to the touch.
Step 2Dump the dough onto a lightly floured work surface, and knead until very smooth, about 1 minute, sprinkling just enough flour so you can roll the dough into a smooth ball. Wrap the dough in plastic wrap, and set aside to let rest at room temperature for 30 minutes. (Dough can also be made a day ahead and refrigerated; let come to room temperature before rolling.)
Step 1Bring a large pot of salted water to a boil. Cut the leaves from the stems of the chard and cut into 1-inch strips. Cut the stems into 1/2-inch pieces and keep separate. When the water boils, add the stems and boil for 10 minutes, then add the leaves and boil until both are tender, about 15 minutes more. Drain, let cool, then squeeze in your hands until most of the water is out. Chop and set aside.
Step 2Meanwhile, put the potatoes in another pot with water to cover and simmer until tender enough to pierce with a fork, about 30 minutes. Drain. When they are cool enough to handle, return the potatoes to the pot and mash, adding the cream and olive oil. Add the chopped Swiss chard and mix well. Beat the eggs and salt together and mix into the potato-chard mixture. Fold in the mozzarella and Grana Padano and set aside.
Step 3Heat the oven to 375 degrees. On a floured surface, roll the dough to fit an 18-by-13-inch rimmed baking sheet with about 3 inches extra on all sides, trimming if necessary. Butter the pan. Fit the dough onto the sheet pan, with the extra dough hanging off the sides, and spread the filling evenly over the dough. Fold the overlap of the dough over to form a 2-inch crust around the pan over the filling, leaving the center without crust. Bake until the filling is set and the crust is golden, 40 to 45 minutes. Cool on a rack. Serve slightly warm or at room temperature.