Put the sweet potato in a small saucepan, add boiling water just to cover, then stir in a teaspoon and a half of salt and cook on medium-high heat for 10-12 minutes, until soft enough to mash. Drain the potatoes, add a tablespoon of oil, and mash until smooth. Leave to cool slightly, then stir in half the lime zest and two tablespoons of yoghurt.
White or golden-fleshed sweet potatoes are more starchy than orange ones, and have a nuttier, slightly drier texture that makes them ideally suited for fritters (a soggy mix doesn’t lend itself to frying). Purple-fleshed sweet potatoes, meanwhile, are a wonderful recent revelation for me.
Drain; transfer to a mixing bowl and mash well with a potato masher. To the mashed sweet potatoes in the bowl, add the mozzarella, parmesan, green onions, 1/2 cup of bread crumbs, garlic, parsley and egg. Season with salt and pepper, and mash all ingredients together well with a potato masher to combine.
While the sweet potatoes are in the oven, roll out the puff pastry to about 1/16 inch (2 mm) thick on a lightly floured work surface. Cut out four 2 3/4 by 5 1/2-inch (7 by 14-cm) rectangles and prick them all over with a fork.