Make the dough: In a large bowl, combine the flour and salt. Add the warm water and olive oil and mix until a dough forms. Knead the dough for 5 minutes until smooth and elastic. Cover and let rest for 30 minutes.
Make the filling: In a skillet, sauté the chard, onion, and garlic until the chard is wilted and the onion is translucent, about 5 minutes. Let cool slightly, then transfer to a bowl. Stir in the ricotta, Parmesan, egg, nutmeg, salt, and pepper.
Preheat the oven to 375°F (190°C).
On a lightly floured surface, roll the dough out to ⅛-inch thickness. Cut into 4-inch rounds using a biscuit cutter or the rim of a glass. Place a heaping tablespoon of the filling in the center of each round. Fold the dough over to form a half-moon shape and crimp the edges to seal.
Place the barbagiuan on a parchment-lined baking sheet. Bake for 20-25 minutes, until golden brown. Serve hot.
