For the bread, in a large bowl mix half the flour and half the yeast and 475ml/17fl oz lukewarm water to form a batter. Set aside in a warm room for 30 minutes.
When the starter is risen and bubbling, add the remaining flour, yeast, salt and oil. Mix until all the dry ingredients have been incorporated.
Turn out the dough onto an oiled work surface and knead for 10 minutes, or until the dough is smooth and elastic.
Oil a large mixing bowl and add the dough. Cover with cling film and set aside in a warm place for an hour to prove, it should roughly double in size.
Meanwhile, make the filling for the bread. Heat a little olive oil in a frying pan. When hot, add the onion and cook until it starts to brown. Remove from the heat and add the tomato pureé, harissa, saffron and four tablespoons of water. Stir to make a paste, then set aside to cool.
For the bread, when the dough has doubled in size, lightly flour a work surface. Turn the proved dough out and divide into four equal portions (use a dough scraper or a serrated bread knife to help you).
Roll out the dough into long rectangle shapes, approximately 10x50cm/4x20in and about 5mm/¼in thick.
Spread the filling over the dough, leaving a 2cm/¾in border round the edge. Top with the olives and goats’ cheese.
Tightly roll up the dough portions lengthways to form sausage shapes about 50cm/20in long. Using your fingers pinch all along the join to make sure the filling stays inside.
The rolls of dough can now be lengthened by sketching the dough until it is about 80cm/32in long.
To form the plait, lay the four pieces of shaped dough on a lightly floured work surface adjacent to each other. Join the strands of dough at one end. Now you are ready to plait the dough.
Number the strands of dough 1-4 from left to right. Put strand four over into position two, then move strand one over into position three, finally put strand two over three. Repeat until the plait is finished.
Oil and lightly flour a large flat baking tray. Place a chefs’ ring, about 15cm/6in in diameter, in the centre and grease the outside with a little oil (if you don’t have a chef’s ring you can use a small ovenproof dish instead).
Place the dough on the baking tray and wrap it around the metal ring. Join the ends together to form a ring. Cover loosely and set aside to prove again for approximately 45 minutes, or until the dough has increased in size.
Meanwhile make the bessara dip. Place all the ingredients and four tablespoons of water in a food processor and blend until smooth (the consistency should be quite watery). Pour the dip into a saucepan and set aside.
Preheat the oven to 220C/425F/Gas 7.
When the plait has proved, brush it with the beaten egg and bake for 20 minutes, then reduce the temperature to 200C/400F/Gas 6 and bake for a further 25 minutes.
Remove from the oven and cool on a wire rack.
While the bread is cooling, warm the dip in the saucepan.
To serve, place the bread on a board with a small bowl (or tagine) in the centre. Pour the warm dip into the bowl. Decorate the bread and dip with some chopped mint.
The dip tastes just as nice cold, so there's no need to heat it and it can be made in advance if you prefer.