Put the onion in a bowl and use your fingers to gently separate the rings. Add a pinch of salt, the elderflower cordial and red wine vinegar and mix to coat, then set aside for about 30 minutes ...
To make the fennel vinaigrette, heat the oil in a small saucepan over medium heat, until it reaches 80˚C. Whisk in the anchovies, followed by the remaining ingredients (except the lemon juice).