Instructions

For the crust: Grate half stick butter using coarse holes on box grater and place in freezer. Cut remaining 2 sticks butter into ½-inch pieces.
Pulse 1½ cups flour and salt in food processor until combined, about 4 pulses. Add butter pieces and process until homogenous dough forms, about 30 seconds. Using hands, carefully break dough into 2-inch chunks and redistribute evenly around processor blade. Add remaining 1 cup flour and pulse until mixture is broken into pieces no larger than 1 inch (most pieces will be much smaller), 4 to 5 pulses. Empty mixture into medium bowl. Add grated butter and toss until butter pieces are separated and coated with flour.
Sprinkle ¼ cup ice water over mixture. Toss with rubber spatula until mixture is evenly moistened. Sprinkle remaining ¼ cup ice water over mixture and toss to combine. Press dough with spatula until dough sticks together. Divide dough in half and transfer to sheets of plastic wrap. Draw edges of plastic wrap over first dough half and press firmly on sides and top to form compact, fissure-free mass. Flatten to form 5-inch square. Repeat with second dough half. Refrigerate for at least 2 hours or up to 2 days. Let chilled dough sit on counter to soften slightly, about 10 minutes, before rolling.
For the filling: Adjust oven rack to lower-middle position and heat oven to 400 degrees. Cook oil and ⅓ cup pancetta in Dutch oven over medium-low heat until pancetta is browned and fat is rendered, 5 to 7 minutes. Using slotted spoon, transfer pancetta to bowl. Pour off all but 1 tablespoon fat from pot.
Add onion to fat left in pot and cook over medium heat until softened, about 5 minutes. Stir in garlic and cook until fragrant, about 30 seconds. Increase heat to high; add chard, 1 handful at a time, and cook until beginning to wilt, about 1 minute. Cover and continue to cook, stirring occasionally, until chard is wilted but still bright green, 2 to 4 minutes. Uncover and continue to cook until liquid evaporates, about 5 minutes. Transfer chard to large bowl and let cool to room temperature, about 30 minutes.
Grease rimmed baking sheet. Stir Parmigiano, ricotta, if using, and pancetta into chard. Roll 1 dough square into 14 by 10-inch rectangle on well-floured counter. Loosely roll dough around rolling pin and unroll it onto prepared sheet. Spread chard mixture evenly over crust, leaving 1-inch border around edges. Brush edges of crust with egg.
Roll remaining dough square into 14 by 10-inch rectangle on lightly floured counter. Loosely roll dough around rolling pin and unroll it over filling. Press edges of crusts together to seal. Roll edges inward and use your fingers to crimp. Using sharp knife, cut through top crust into 12 equal squares (do not cut through filling). Brush with remaining egg and sprinkle with remaining ⅓ cup pancetta.
Bake until pie is golden brown and pancetta is crisp, 30 to 35 minutes, rotating sheet halfway through baking. Transfer sheet to wire rack and let pie cool completely, about 30 minutes. Transfer pie to cutting board, cut into squares, and serve.