Method:
Bring a large pot of salted water to boil. Add swiss chard and blanch for 3 to 4 minutes, or until tender. Drain and run under cold water to cool. Squeeze out as much excess water as possible, then transfer chard to a large bowl. Mix in onions, potato and cheeses. Add oregano, salt and pepper, then add eggs and stir until combined. Set aside.
Preheat oven to 400°F.
On a lightly floured workspace, unfold puff pastry and cut in half. Roll one half into a 9-inch circle, sprinkling it lightly with flour to avoid sticking. Transfer circle of dough to a nonstick baking sheet and pile swiss chard mixture in the center, gently spreading it out and leaving a 1/2-inch border of pastry around the entire circle. Roll out the remaining puff pastry into a 10-inch circle and gently place on top. Smooth out and tuck the overlapping pastry completely under the bottom circle. With a sharp knife, poke a few holes on top. Lightly brush with olive oil and bake 35 to 40 minutes, or until golden. Cut into wedges.