For the dipping sauce, fry the onion and garlic in the olive oil in a pan until the onions are translucent.
Add the spices and the tomato purée cook for a further minute or two. Add the chicken stock and the rest of the ingredients. Cook for 10 minutes. Taste and add more salt or pepper if necessary. Set aside to cool.
Preheat the oven to 130C/280F/Gas 1.
For the oysters, place the flour in a bowl, add a pinch of salt and then gradually mix in the iced water until the batter thinly coats the back of a spoon. Squeeze in a little lemon juice.
For the oysters, shuck the oysters and then lay them on kitchen paper to dry.
Heat the vegetable oil in a deep fat fryer or a heavy-based saucepan to 180C/350F. (CAUTION: hot oil can be dangerous. Do not leave unattended.)
Dip the oysters in the batter and then carefully place into the hot oil in batches of 4-6, depending on the size of the oysters. No more or it will cool the oil too much. Remove with a slotted spoon and transfer to a tray lined with kitchen paper. Keep warm in a low oven until all are ready.
Serve the oysters with the chilli dipping sauce.