- Sift the flour into a large bowl and season with salt and pepper.
- Add the butter and rub into the flour using your fingertips until the mixture resembles fine breadcrumbs.
- Sprinkle with 6-7 tablespoons chilled water (enough to just bind the dough) and draw the mixture together with a knife.
- Knead gently on a floured board then wrap in cling film and chill for 30 minutes in the fridge.
- Heat oven to 220°C/fan 200°C/425°F/gas 7.
- Heat the oil and add the bacon and onion, cook for 4 minutes.
- Add the sweet potato, turn up the heat and fry for 5 minutes, stirring occasionally, until the bacon is crisp and the potato just underdone.
- Add the thyme and chicken.
- Leave to cool slightly.
- Unroll the pastry and with a rolling pin, roll out until it's large enough to cut out 4 circles of pastry, using a large saucer or a side plate as a guide.
- Discard the trimmings.
- Lift the circles onto a baking sheet.