1lb frozen puff pastry, defrosted in the fridge and rolled out
Oil for frying
1oz butter
2lb onions, finely sliced
2 tbsp fresh thyme leaves, plus extra to serve
2 tbsp Dijon mustard
6oz anchovy fillets, drained (optional)
16-20 black or green olives, pitted and halved
Preheat the oven to 400F.
Heat the oil and butter in a very large frying pan over a medium heat until bubbling. Add the sliced onions with a pinch of salt and cook for 30-40 minutes, stirring occasionally, until completely softened and caramelized.
Once the onions have caramelized, stir in the thyme leaves and cook for 1 minute more, then transfer to a bowl to cool slightly.
Using a sharp knife, score a border 1.5cm from the edge of the pastry. Brush the mustard over the pastry inside the border, then spoon over the onions and spread evenly inside the border. Arrange the anchovies (if using) over the onions in a lattice, creating a diamond pattern, then put an olive half in the middle of each diamond. Bake for 25-30 minutes until golden and crisp, then transfer to a board, scatter with fresh thyme leaves. Cut into pieces and serve right away.
If serving with arugula, you can add a simple vinaigrette of olive oil, lemon juice, salt and pepper to the lettuce before serving.
Serves 8 as a starter or many as a canape