Bring a medium pot of water to the boil. Prepare in advanced a bowl of iced water and a slotted spoon.
Once the water is boiling add in the spinach and leave for 30 seconds. Remove the spinach with the slotted spoon and plunge it straight into the iced water. Repeat this process with the parsley. This allows the spinach and parsley to slightly break down but retain its flavour and green colour.
Squeeze the spinach and parsley to remove as much water as possible and roughly chop. Place the chopped spinach and parsley into a blender.
Melt the butter in a pan and add the flour. Stir consistently until a paste forms. Add your milk and whisk until smooth.
Add in your confit garlic, chicken stock (if you’re using this sauce on fish, you can use fish stock here) and Dijon mustard. Whisk again until smooth.Salt to taste.
Pour this butter emulsion into the blender. Blend on high speed until the herbs have broken down and have formed a bright green puree. Pour through a fine mesh sieve and use the back of a spoon to push as much puree through as possible.
Season to taste and add a little lemon juice to brighten the sauce up.
