(Pictured) Serves 6.
For the filling, you will need:
900g (2lbs) onions, thinly sliced 3 tablespoons olive oil 2 sprigs fresh thyme 2 sprigs fresh rosemary Sea salt and freshly ground black pepper
For the pastry, you will need:
225g (8oz) flour, plus a little extra to flour the work surface 110g (4oz) butter 1 egg, beaten
For the topping, you will need:
12-16 black olives 2 x 50g (1 ¾ oz) tins of anchovy fillets, drained First, cook the onions. Pour the olive oil into a wide pan on a medium heat and allow it to get warm. Tip in the thinly sliced onions, the fresh thyme and the fresh rosemary sprigs, season with sea salt and freshly ground black pepper, mix well, and then turn the heat down to medium-low. Cook the onions, uncovered, stirring regularly, for about 30-45 minutes until they are meltingly tender and a light golden brown.
While the onions are cooking, make the pastry. Sift the flour into a bowl, and rub in the chilled butter until the mixture resembles coarse breadcrumbs. Next, bring the mixture together with half or three-quarters of the beaten egg — but you may need it all. The pastry dough should not be wet and sticky, so don’t add all the beaten egg if you don’t need to. Flatten the pastry to about 1cm thick (less than ½ in), cover it with cling film and put it in the fridge to chill for 20 minutes.
Preheat the oven to 180°C, 350°F, Gas 4. While you’re waiting for the oven to preheat and the pastry to chill, you can prepare the black olives by removing the stones. You may need to bash them slightly with the flat side of a knife to help loosen the stones.
When the pastry has chilled, place it on a floured work surface, and dust a little flour on top of the pastry also. Roll it out into a rectangle that’s large enough to line the base and sides of a small Swissroll tin, approximately 23cm x 30cm (9in x 12in) in size. Brush off any excess flour, then, with the help of the rolling pin, carefully transfer the pastry into the Swiss roll-tin (there’s no need to grease or line the tin). Trim the pastry all around the sides at the top, ensuring that the tin’s sides and base are lined with pastry. Remove the thyme and rosemary sprigs from the cooled onion mixture and tip the cooked onions onto the pastry. Run a spoon or a palette knife over the top to ensure the onions are smooth and level. Arrange the anchovy fillets in a criss-cross pattern, as in the photo, above, then place an olive inside each diamond shape.
Put the tart in the preheated oven and bake it for about 30 minutes, until the pastry around the sides is golden. Cut it into slices and serve it hot, or at room temperature.