- Preheat oven to 200°C/400°F/Gas 6. Grease a 9”/23cm round baking sheet.
- Melt 25g/1oz of the butter in a small pan and fry onion until beginning to brown. Add in half the thyme.
- Place flour and remaining butter in a bowl and rub in until it resembles breadcrumbs.
- Add the onion and juices from the pan, and the mashed potato.
- Mix to form a soft dough, then spread out onto prepared baking sheet.
- Arrange tomatoes on top of dough, drizzle with vinegar (if used) and sprinkle with remaining thyme. Place pieces of cheese on top and drizzle over the olive oil.
- Place in preheated oven and bake for 35-40 minutes.
Serve with a simple green salad