Bring classic French flavour to summer picnics with this light onion tart. Originating in Nice, pissaladière is traditionally topped with onions, slow-cooked until gorgeously caramelised, black olives and anchovies. We've swapped out the anchovies to make our version vegan-friendly, but the rich, sticky-sweet flavours in this picnic bake still deliver the same moreish results.
Unroll the pastry sheet on a chopping board. Score a border, 2cm (1in) from the pastry edge, with a sharp knife. Prick the inner rectangle all over with a fork. Transfer to a large baking sheet and chill until needed.
Put the olive oil in a large casserole or saucepan with a lid. Set over a low-medium heat and add the onions and leeks with a large pinch of salt. Cook, stirring for 5 minutes, then cover with the lid and leave to cook for 25 minutes, stirring occasionally and reducing the heat if the mixture catches. The onions should be very soft with liquid in the pan.
Remove the lid and turn the heat up to medium-high and add the sugar and half of the thyme leaves. Cook the onion mixture, stirring constantly, until caramelised and reduced in volume with no liquid remaining. Set aside to cool. This step can be done up to 3 days in advance. Keep the cooked onions chilled and covered until needed.
Preheat the oven to gas 5, 190°C, fan 170°C and place a large baking tray inside to heat for 5 minutes. Spread the onion mixture out evenly over the inner pastry rectangle. Stud with black olives at regular intervals.
Remove the baking tray and place the pastry on top, baking for 20-25 minutes, or until the pastry border is puffed and golden brown. Allow to cool for at least 15 minutes before serving warm or completely cool. Scatter with the remaining thyme and rocket leaves to finish.
Tip: Serve with extra salad at a summer picnic or cut into smaller squares and serve as a party nibble.
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