Heat a drizzle of oil in a large non-stick pan. Add the red onion and fry on a medium heat for 5 minutes.
Crumble both blocks of feta into a large mixing bowl. Add the fresh breadcrumbs, dried Thyme, chopped fresh spinach, honey, 2 eggs, fried onion and milk. Season with ground black pepper.
Give it a really good stir, working the milk and the egg through the ingredients until it is all absorbed and everything is thoroughly combined.
Lay the puff pastry out on the worktop.
Place the filling lengthways down the centre of the pastry - leave around a 5cm gap of pastry either side of the filling and around 3cm of pastry either end.
Flatten the filling out a little and make sure it's evenly spread.
Starting in the top left corner cut 1cm diagonal strips into the pastry from the filling outwards.
Tuck the ends of the pastry over the filing. Fold the strips of pastry alternately into the middle crossing over each other.
When all the pastry is folded up and the filling is encased, brush with beaten egg. Bake at 200℃/400℉ for 25-30 mins