Heat the oil in a frying pan and gently fry the onion until softened.
Add the stock and simmer until reduced by two thirds.
Add the lemon juice, zest and crème fraiche and simmer for 5 minutes.
Place the white chocolate in a large bowl and pour in the onion mixture, whisking well until the chocolate has melted and the mixture is smooth and glossy.
Add the vermouth, herbs, pepper and salt and stir to mix.
Serve immediately or store in the fridge until needed.