To make the mayonnaise, place the yolk, vinegar, mustard, chilli flakes, chives and hot water into a food processor and blend.
Whilst the food processor is running, gradually add the vegetable oil in a constant flow.
When all the vegetable oil has been added, season with salt and freshly ground black pepper.
To make the remoulade, place the fennel, shallot, parsley and lemon juice into a large bowl and mix well.
Add some of the mayonnaise and mix once more to coat, then season, to taste, with salt and freshly ground black pepper.
To serve, spoon the remoulade into the centre of a plate and garnish with the chervil leaf.