Step 1

Melt 50g of butter in a pan over a medium heat, then sprinkle over the flour and stir vigorously until you have a smooth paste. Add the milk, a splash at a time, stirring continuously until you have a smooth white sauce. Add the bay, tarragon, gruyère and mustard, and cook for 2-3 minutes or until thickened. Remove from the heat.

Step 2

In a separate pan, heat the tbsp of butter and fry the leeks for 15 minutes until very soft – add a splash of water during cooking to stop them catching, if needed. Add the wine and simmer until evaporated. Stir into the white sauce along with the lobster meat and brandy. Cover and chill for 3 hours, or overnight, until set solid.

Step 3

Steam the spinach until wilted, then cool until you can handle it. Tip into a clean tea towel, wrap up and squeeze as much water as possible from the spinach.

Step 4

Cut off a third of the pastry and roll out on a well-floured worksurface into a 20cm x 30cm rectangle. Transfer to a heavy baking tray lined with baking paper. Arrange the spinach over the pastry rectangle, leaving a 4cm border around the edge, then spoon the lobster filling down the middle of the spinach.

Step 5

Roll out the remaining pastry as before into a 40cm x 30cm rectangle. Brush some of the beaten egg around the exposed pastry edges of the filled rectangle, then lay the larger rectangle on top. Crimp the edges with a fork or your fingers to enclose the filling and seal. Trim the excess and use the off-cuts to decorate the top, if you like. Brush the pastry all over with most of the remaining beaten egg, cut a large steam hole in the top and chill for 30 minutes.

Step 6

Heat the oven to 220C/fan 200C/gas Glaze the pastry with any remaining beaten egg and bake for 45-50 minutes or until golden, puffed, crisp-based (lift a corner to check) and piping hot throughout. Leave to rest for 10 minutes before carving.