Heat a deep fat fryer or heavy-based saucepan of vegetable oil to 190°C.
Whisk the yeast and beer together in a bowl.
Whisk in 200g of plain flour until smooth and set aside for five minutes.
Shuck the oysters and collect their juice in a separate bowl.
Dust the oysters in the remaining 100g of flour.
Dip each oyster into the batter and drop carefully into the hot oil.
Fry for two minutes until golden-brown and drain on kitchen paper.
Boil the vinegar and sugar in a saucepan until the sugar dissolves.
Pour the hot vinegar over the sliced onion and chilli and let sit for five minutes.
Whisk two tablespoons of pickling liquid, two tablespoons of oyster juice, rapeseed oil, salt, and pepper.
Toss the watercress and drained pickled onions with the prepared dressing.
Serve the salad topped with six oysters per plate and a pinch of smoked sea salt.