A recipe from my friend, the chef Matt Tebbutt. You might think that the sweetness of caramelised onions, puff pastry and sardine flesh would be too much together, but this is actually very good and kind of looks like a Mediterranean stargazy pie. If you want a bit of contrast, lay some salty anchovies on top.
6 sardines, gutted, cleaned and filleted (heads removed but tails left on)
On a sheet of greaseproof paper roll out the pastry to a 30cm circle. Slightly pull up the edges all the way round to make a rustic-looking crimped rim. Prick all over with a fork and put in the fridge to chill.
Put the onion and olive oil in a large sauté or frying-pan over a medium heat. Start to fry the onions, adding a pinch of salt when they begin cooking, then turn the heat down low and slowly cook until the onions are caramelised – about an hour. Keep an eye on the pan and turn the onions over every so often. Add a splash of water every now and then to stop the onions catching and burning on the bottom. Add the leaves from the sprigs of thyme and season to taste.
Preheat the oven to 200C/190C fan/Gas 6. Put a metal baking sheet, big enough to fit the tart, into the oven to heat.
Slide the pastry onto the hot baking sheet and cook for 20 minutes. Flatten the top if it puffs up.
Remove the base from the oven and spread the onions over, leaving a border of 5cm. Scatter the olives on top. Lay the sardine fillets on top like the spokes of a wheel, the tails at the edges. Put the tart back in the oven and cook for a further 15 minutes, or until the sardines are cooked through. Serve at once.