Preheat the oven to 200°C/180°C fan (400°F/350°F).
Prick the sweet potato several times with a fork and cook in the microwave until soft - around 5-6 minutes, depending on size.
Heat the olive oil in a large pan on a medium heat and add in the onion. Cook until soft, stirring regularly. Add in the garlic and cook for another 2 minutes.
Add in the spinach and stir until it has just wilted.
Once the potato has cooled, peel it and place in a large mixing bowl. Mash the potato with a fork until smooth.
Add the spinach mixture into the mixing bowl and season with salt and pepper. Mix with a fork.
Fold the feta cubes into the potato mix.
Slice a the ready-rolled puff pastry into thirds width ways. Place a third of the mixture onto one half of each strip.
Fold over the pastry and crimp the edges using a fork. Use a knife to slice some small steam holes into the pastries.
Brush each pastry with the beaten egg and sprinkle over some mixed seeds.
Bake for 15 minutes or until golden brown.
