Shuck each oyster and discard the top shells. Turn the oysters, detaching them from the lower shell and pour the liquor into a jug and reserve for later.
Chop the fennel, celery, onion and garlic and add to a pan with a little olive oil. Gently fry for about 10 minutes until softened.
Add the Thai basil, removing larger thicker stalks and add the reserved oyster liquor through a sieve to remove any fragments of shell. Cook the mixture until the liquid has disappeared.
Add the butter, lime juice, Sriracha sauce and Worcestershire sauce and stir until the butter has melted.
Pour the mixture into a beaker, add the breadcrumbs and use a stick blender to process until smooth.
Place the oysters on a baking tray and spoon equal amounts of sauce over each oyster and spread to the edges (optional). Grate a layer of fresh parmesan over each oyster.
Place the oysters under a preheated high grill for 7 minutes. Garnish with any remaining Thai basil and serve.