Preheat the oven to 210°C/190°C Fan/Gas Mark 7.
Place the flour, butter and salt into a food processor and blitz until broken up like breadcrumbs. Add in the fromage frais and blitz briefly until just combined. Transfer mixture into clingfilm and freeze in a wide, thin disc for 15 minutes.
Meanwhile, prepare the filling. Place the spinach in a sieve and pour over boiling water until it has defrosted. Allow to cool and then place it in j-cloth and squeeze as much liquid out as possible. Place in a bowl and add all the other filling ingredients, mix well and season with salt and pepper.
Once the pastry has chilled, flour the surface well and roll out to 3 – 4mm thickness (roughly a 50 x 25cm rectangle). Use a pizza cutter to divide into 3 20cm x 15cm rectangles.
Starting with the first rectangle, lay it horizontally and take a third of the filling and lay it in a fat sausage down the centre. Use a knife to make diagonal strips at 1 cm increments stopping at the filling. With the last strip on both sides, cut the strips completely off and discard. You should be left with a flap at the bottom that you can fold up over the bottom of the filling. Working from the top, fold the first left strip over diagonally and under the filling like you are wrapping the filling up and then with the first right strip. Repeat with the other strips until the last two strips can be tucked underneath the pastry to neaten.
Repeat with the other two plaits and then transfer to a greaseproof lined tray.
Brush the plaits with egg and then sprinkle over the fennel seeds.
Bake for 20 minutes or until golden.