Preheat the oven to 180°C fan (200°C conventional) and lightly grease a cast-iron skillet or round baking dish.
Heat the olive oil in a large frying pan over a medium heat. Add the onions and cook for 8–10 minutes until softened and lightly golden.
Add the garlic and cook for a further minute until fragrant.
Stir in the mixed greens and cook for 2–3 minutes until wilted. Remove from the heat and allow to cool slightly.
Add the chopped herbs, crumbled feta and lemon zest. Season with salt and black pepper, then mix well.
Unroll the puff pastry and cut it in half lengthways to create two long strips.
Divide the filling evenly between the pastry strips, placing it in a line along the centre of each strip. Fold the pastry over the filling and seal the edges to create two long sausage shapes.
Starting from the centre of the prepared dish, coil one pastry sausage into a spiral. Continue with the second sausage, wrapping it around the first to create one large spiral.
Brush the pastry generously with olive oil and sprinkle over the sesame seeds.
Bake for 30 minutes, or until the pastry is golden brown, crisp and cooked through.
Leave to cool for 5 minutes before slicing and serving.