I have acquired the recipe.
Mr Ferrari's mystical green elixir
or Watercress sauce is the new ketchup.
You will need
3 cloves of garlic
2 Tbsp lemon juice
200g melted unsalted butter
Blitz greenery and garlic, add lemon juice and mayonnaise, pour in melted butter.
Serve with poached salmon, roast chicken, chips, or just dunk a piece of bread in it.
How'd you learn to do the things you do?