Fry onion and garlic in olive oil until translucent.
Stir in spices and tomato purée and cook for two minutes.
Add chicken stock and remaining sauce ingredients and simmer for 10 minutes.
Preheat the oven to 130°C.
Whisk flour, salt, iced water, and lemon juice into a thin batter.
Shuck oysters and pat dry with kitchen paper.
Heat vegetable oil to 180°C in a deep fryer or heavy pan.
Coat oysters in batter and fry in batches of 4 to 6 until golden.
Drain oysters on kitchen paper and keep warm in the oven before serving with sauce.