In a large sauté pan with a lid, on a medium heat, gently sweat the onions in the butter with the salt and pepper for 15 minutes until they become soft.
Remove the lid, increase the heat to high and continue to cook for a further 10 minutes, stirring from time to time to prevent them from sticking.
As the onions begin to caramelise, reduce the heat to low and stir more frequently.
Once the onions have turned a light golden caramel colour, add the garlic, rosemary and thyme, cook for 1 minute, then add the sherry vinegar and continue to cook until the vinegar has completely reduced.
Set the onions to one side and allow to cool.
Pre-heat the oven to 180°C.
In a large bowl, mix together the breadcrumbs, pork mince, sausage meat and harissa paste. Add one-third of the now cooled caramelised onions and mix until they are evenly distributed throughout.
Roll the sausage mixture in cling film the same length as the puff pastry, roughly 25cm and place in the fridge while you prepare the pastry.
Using a rolling pin, roll the pastry so the width is at least 15cm and the length is 30cm.
Using a spoon, spread the Saint Agur Blue Crème evenly along each length of the pastry, leaving a 5cm gap running down the centre for the caramelised onions.
Place the remaining caramelised onions down the centre of the pastry, so the Saint Agur Blue Crème is either side of it. Make sure you leave a 2cm gap at each end.
Remove the clingfilm from the sausage mixture and place it on top of the caramelised onions.
Using a sharp knife, cut diagonal strips 5cm long and spaced 3cm apart down each length of the pastry, making sure they are all facing the same direction. These will form the plait effect when you wrap it over the sausage mixture.
Fold either end of the pastry over the sausage mixture and then plait alternative strips of the pastry over the sausage filling to neatly cover the entire length.
Carefully place the sausage plait onto a baking tray lined with greaseproof paper.
Using a pastry brush, lightly coat the sausage plait with the beaten egg and bake in the oven for 40 minutes until golden brown.
Allow to cool for 15 minutes to cool slightly before you serve, or leave it to cool completely and store in the fridge until you’re ready to go on your picnic.