To make the pastry

1. Divide the butter into four portions and dice each portion into 1cm cubes. In a large mixing bowl, combine the flour, one portion of butter and 135ml of water. Work the dough into large crumbs using the fingertips of both hands. If the dough is dry, add the remaining 15ml of water. Press the dough together firmly, wrap in cling film and chill for 15 minutes.

2. Roll the dough out into a rectangle and sprinkle the second portion of diced butter over two-thirds of the dough leaving an empty square at one end. Fold the non-buttered third over the centre third and fold over the remaining third of the pastry. This rolling and folding is called a "turn". Wrap and chill the pastry for another 15 minutes before rolling out and repeating two more times to use up the butter portions. Wrap with cling film and refrigerate the pastry until needed.

To make the tart

1. Halve the onions vertically, then place each half cut-side down on the chopping board and slice across very thinly, into half moons.

2. In a wide, heavy-based saucepan, simmer the sliced onions, olive oil and a pinch of salt over a very low heat. Stir them occasionally as they release their juices, and then more frequently as the liquid evaporates and the onions start to fry again. Don't let them colour, though – you want the onions to be melty, translucent and pale. This takes about 30 minutes. Once the onions are cooked, set them aside.

3. Preheat your oven to 190C (170C fan-forced).

4. On a clean, lightly floured bench, roll the pastry out to 5mm thick and cut out a rectangle about 13cm x 40cm. Spread 5mm thick layer of cooled onions over the pastry, leaving a 1cm border all around. If the olives are pitted, cut them in half. If they are not pitted, slice "cheeks" of olive away from each pip. Place anchovy fillets across the tart at regular intervals and place two halves or "cheeks" of olive between each anchovy.

5. Slide the tart onto a baking tray and bake for 20 to 30 minutes, until the base is well crisped. Cut into wedges and serve immediately.

Serving suggestion: with my red pepper salad.

Find more of Andrew McConnell's recipes in the Good Food Favourite Recipes cookbook.