Preheat the oven to 200°C/400°F/Gas Mark 6.
Melt the butter over a medium heat, remove and stir in the flour to form a roux. Place back on the heat and cook for 30 secs stirring with a wooden spoon.
Gradually add the milk a little at a time whisking as you go to prevent any lumps from forming. When you have incorporated all the milk and the sauce has a creamy consistency add in the Dijon mustard, Pié d’Angloys and a squeeze of lemon juice. Season to taste.
Wash the spinach and cook for 1-2 mins, squeeze out any excess liquid using a sieve and the back of a spoon.
Divide the spinach between two small crockery-baking dishes, place the fish on top and finish with the sauce.
Bake in the oven for 15 to 20 min until golden on top.