1.Preheat the oven to 220°C, fan oven 200°C, Gas Mark 7 and lightly grease a large baking sheet.
2. Unroll the puff pastry sheet onto a lightly floured surface and cut into 6 equal rectangles. Place on the prepared baking sheet.
3. On each rectangle, score a border 1cm from the edge, all the way round, taking care not to cut right through the pastry. Brush the border with beaten egg. Keep chilled while you make the filling.
4. Melt the butter in a large frying pan and cook the red onion until soft, about 10 minutes. Add the Tate & Lyle Dark Soft Brown sugar and balsamic vinegar and cook for another minute or so. Stir in the tomatoes.
5. Spoon the onion and tomato mixture onto the centre of each pastry rectangle. Bake for 20-25 minutes until risen and golden brown. Serve, sprinkled with feta cheese, chopped fresh chives or parsley and freshly ground black pepper.
If you’re making these for a buffet, make in advance, then warm them through just before serving.