First prepare the Filling  for the Salmon en Croute. Peel the cap of every Chestnut Mushroom carefully. Remove the stalks from the Mushrooms and clean them one by one with some kitchen towel of impurities. Chop very thinly all of the Mushrooms (caps and stalks). Season with Black pepper and a little Salt the chopped Mushroom in a bowl then set aside.

Secondly, prepare the Spinach by putting them in a heat proof bowl. Pour the boiling water of a Kettle over the Spinach and wait until the Spinach leaves have wilted. Drain the Spinach. with a wooden spoon tap with gentle pressure the Spinach in order to get rid of any excess water. Set aside to cool down before the next step.

The next step consist to pour your chopped Mushrooms upon the Spinach and to combine them gently together. Whisk the Cream Cheese to a smooth consistency and add it to the mixture. Break an Egg and seperate it in order to have just the Yolk which you will add to the bowl, however keep the Egg White apart for later. Season a little before combining carefully everything together. Once this is done cover with cling film and set aside.

Concentrate on preparing the Salmon. It is best to get the Salmon Fillets prepared by the Fishmonger but also easier. The Fishmonger will skin the Fish for you but also debone it. You will also have some nice evenly weighted Fillets to deal with. So the harduous task and time consuming one will already be out off the way.

To prepare your Salmon put the Fillets upon a Board, a Fish Board if you have one, it is blue. It is very useful in a Kitchen to have colour coded Boards to prevent cross contamination. It goes like that: Blue for Raw Fish, Red for Raw Meat, Green for Salad and Fruits, Yellow for Cooked Meat, Brown for Vegetables, White for Bakery and Dairy. Possessing a set myself I found it practical. 

Season your Salmon Fillets with Sea Salt and cracked Black Peppercorns. Chop some Dill to scatter all across the Fillets. Once this is done then you can spread the Mushroom and Spinach mixture upon one of the fillets evenly.

Break the second Egg in a small Bowl, add the White of the first Egg and pour 1 tablespoon of Milk if not two, before whisking until you have a silky smooth mixture. This is the Egg Wash which will be brushed onto the Pastry later in order to have a Glaze.

As for the Pastry, I tend to buy my Puff Pastry rather than making it from scratch, it simply just save time and energy. Cut a small slice from the block to keep aside, it will be used for decoration later upon the Salmon en Croute. Wrap the small part in cling film and put it back in the Fridge.

On a floured surface roll the larger part of the Puff Pastry. The aim is to make sure that it is long enough and wide enough to cover the entire Salmon Fillets. One of the hardest thing to do which I do find tricky and not simple is to place the rolled up pastry on a baking sheet without breaking it. But when it is done place the Salmon Fillet covered by the Mushroom and Spinach Spread first in the centre of the Pastry, then top it with the Fillet without the Spread. This way it will create three layers with the Filling being the one in the middle.

You will need to brush the Pastry all around the edge of the Salmon with the Egg Wash. Then the folding part start in earnest. Start by the ends parts of the Fillet and fold them until the top edge of the Fillet. Brush with the Egg Wash the top end bits of the Puff Pastry. It will act as a sort of glue in that instance. Then turn your attention to the length sides of the Salmon and bring the Pastry upward on both sidesup until they meet in the middle covering then the entire Salmon. Pinch where the two flap of Pastry meet, to seal them properly brush with the Egg Wash. It will form a sort of crest which you can pattern with the tips of your fingers.

Once this is done put your Salmon en Croute in the Fridge for about 30 minutes give or take. Whilst in the Fridge you can start to roll out the remainder of the Puff Pastry. Reapply some Flour on the surface if needed. This time it will be to do some decoration which may take time to conceive or create, so before you start doing the shapes of Pastry, preheat the oven at Gas mark 7.  However that decorative step can be rendered simpler if you invest of some pretty shape Kitchen cutters. They are useful. My set is of Leaves of different kinds: Oak, Maple, Holly, common shaped Leaf. 

Remove the Salmon en Croute from the Fridge and apply your shape with the Egg Wash the shapes you created upon or near the crest, then finish off by lavishing the rest of the Egg Wash upon your Salmon en Croute before putting it on a Baking tray then in the oven. Approximatively it should bake for 40 to 45 minutes to be well done. Usually you can tell if it is done by the colour of the Pastry, if it is a nice golden colour then it will most probably be ready.

After taking the Salmon en Croute out of the oven let it rest just like you would do for a steak. It allows the crust to firm up a little for 15 minutes which will make cutting the Salmon en croute easier to cut.

Meanwhile you can prepare a small side by sautéing the peeled and chopped Shallots, slice Spring Onions and a couple of chopped Garlic Cloves in a small knob of Butter. Then add 250 ml of White Wine, 250 ml of Veg Stock, before putting the Peas in. The Peas do not take long to cook especially if we want for them to not loose their bright Green colour. Finish that Side with some refreshing Mint Leaves.

When this is done you are ready to serve. The Samon en Croute is always a centre piece to be cut in front of the Guests for good effect. I will also advise to serve it with a  gravy Boat full of Watercress Sauce which can be prepared well in advance.