Parisienne Sauce is a room temperature cheese-based sauce that requires no cooking.
It is used for cooked vegetables that are then being served cold or room temperature — classically, asparagus.
Some modern sauces — such as a meat sauce with eggs and cream in it — are trying to hijack the name of Parisienne Sauce, but as nice as it sounds, it needs another name.
2 oz / 50 g of a cream cheese (ideally Gervais, but in practice even Philadephia is fine)
1 tablespoon fresh chervil, chopped
In a food processor, whiz the cheese until soft and smooth. Drizzle in enough olive oil, whizzing as you go, to make it into a spoonable sauce. Stir in chervil by hand (don’t whiz; it’s not meant to be a green-coloured sauce.) Adjust taste with a few squirts of lemon juice, and some salt and paprika. Keep chilled until needed, then bring to room temperature shortly before use.