Bring the white wine to the boil with the onion and reduce to half. Add the melted butter and stir well. Mix the egg yolks with the vinegar using and electrical mixer and slowly add the butter-wine mixture, keep mixing. Finally, add the tarragon and pepper and salt to taste. You can reheat the sauce before use but don't boil.

Béarnaise means "from Béarn", which is a former province of France, situated in the department Pyrénées Atlantiques, together with the French part of Basque Country. The sauce is known to be prepared already in the middle of the 18th century.