Paul says this recipe is one of his absolute favourites – taking him back to the days when he first started working in a bakery. It’s flakey in the mouth, and the cheese combined with a slight hint of mustard gives this recipe a gorgeous kick.
For the cheat’s rough puff, mix the flour and salt together in a bowl. Rub in the chilled butter using your fingertips, until the mixture resembles breadcrumbs.
Gradually add enough water to form a dough (about 6-8 tablespoons of water).
Roll the dough out into a long rectangle (approx. 30cm x 15xm) on a lightly floured work surface.
Coarsely grate half of the frozen butter over the bottom two thirds of the dough. Fold down the top third and fold up the bottom third as if folding a letter.
Turn the folded dough through 90 degrees and roll it out into a rectangle again. Repeat the process of adding the remaining frozen butter and fold as before. Wrap the dough in cling film and leave to rest in the fridge for 30 minutes.
For the filling, heat the oil in a small frying pan over a medium heat, add the onions and cook for 10 minutes until soft and translucent. Remove from the heat & leave to cool.
While the onions are cooking, make a white sauce. Melt the butter in a small pan then add the flour. Stir continuously until a paste forms, then cook for 1 min over a low heat.
Add the milk gradually, stirring continuously, until you have a smooth sauce. Cook for 3 mins over a low heat, stirring continuously, until the sauce has thickened. Add the mustard powder then season with salt and pepper to taste. Remove from the heat.
Add the cooled onions, Cheddar and Mozzarella cheese to the sauce and stir until combined. Spread out on a plate or shallow tray and chill in the fridge for 10 mins.
Roll out the rested dough into a long rectangle and repeat the folding process, folding down the top third and fold up the bottom third as if folding a letter, then roll it out into a rectangle measuring 30cm x 40cm.
Cut out 8 rectangles each measuring 15cm x 10cm.
Place 4 of the rectangles on a baking sheet lined with baking paper. Divide the cheese and onion mixture evenly between the 4 rectangles, leaving a 1cm border of pastry around the edges. Brush some of the beaten egg around the edge of the pastry, then cover the filling with the remaining 4 pastry rectangles.
Press the edges of the pastry together to seal, then crimp with a fork. Make 3 diagonal scores across the top of each pastry parcel with a knife, be careful not to cut all the way through the pastry.
Brush with beaten egg and bake for 30 - 35 minutes at 220°C/200°C fan/425°F/Gas 7 until puffed up and golden brown.