Hold an oyster, wrapped in a heavyweight cloth, with the rounded shell underneath. Push a strong knife, preferably an oyster knife, into the small gap at the hinged end. Twist the knife to sever the muscle and separate the shells.
Discard the top shell. Run the blade of the knife under the oyster to loosen it, holding the shell steady to prevent the juices from running out. Place the oyster in a grill pan, lined with a layer of salt to keep the shells from flopping over, and repeat with the remainder.
Dry-fry the mustard seeds in a frying pan until they start to pop. Add the butter, the shallots and celery and fry for 3 minutes. Add the garlic and a little salt and pepper and fry for a further 2 minutes. Stir in the vinegar, Tabasco sauce and two-thirds of each herb.
Spoon the mixture over the oysters and cook under a preheated grill for 5–8 minutes, or until the oysters are just firm. Serve scattered with the remaining herbs.