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 the 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.