Preheat your oven to 400°F (200°C).

In a skillet over medium heat, drizzle some olive oil and add chopped onions. Cook until translucent, about 5-7 minutes.

Add chopped Swiss chard to the onions and sauté until wilted, about 3-4 minutes.

In a large bowl, mix the sautéed Swiss chard and onions with crumbled feta cheese, beaten eggs, and chopped dill or mint. Stir until well combined.

Roll out your puff pastry onto a greased tart pan, pressing it gently into the corners.

Pour your filling into the pastry shell and spread it evenly. Bake for 25-30 minutes or until golden brown and set.