Peel the onion; wash the herbs, then finely chop them along with the capers, onion, and gherkins. Set everything aside.
Crack the egg and separate the white from the yolk; place the yolk in a bowl; add a pinch of salt, a pinch of pepper, and the mustard. Whisk the mixture and slowly drizzle in the oil.
Add a splash of vinegar along with the chopped herbs, gherkins, and capers. Pour the tartar sauce into a sauceboat and set it aside in the fridge.
Red mullet
Heat up the oil.
Make two shallow cuts on each side of the red mullet; dip them in milk, drain them, and coat them in flour. Place them in very hot frying oil and cook for 7 to 8 minutes.
Remove the stems from the parsley sprigs; wash the sprigs thoroughly and pat them dry; cut the lemons into wedges.
Warm a serving dish and cover it with a white napkin.
When the red mullet are cooked, remove them from the oil. Let them drain, then season with salt. Meanwhile, add the parsley to the oil; let it fry for a moment, then drain it.
Arrange the red mullet on a serving platter; surround them with fried parsley and the lemon wedges. Serve piping hot, accompanied by a sauce boat of tartar sauce.