For the bhajis, tip the flour into a large bowl and add enough water to make a smooth batter (about 100ml/3½fl oz).
Heat the vegetable oil in a frying pan and fry the onions for 5-10 minutes, or until golden-brown.
Stir the cooked onions, sweet potato, chilli and coriander into the batter.
Half-fill a deep, heavy-bottomed saucepan or deep fat fryer with vegetable oil and heat until a small cube of bread sizzles and turns golden-brown when dropped into it. (CAUTION: hot oil can be dangerous. Do not leave unattended.)
Shape the bhaji mixture into small rounds and deep-fry for 2-3 minutes, or until crisp and golden-brown. Carefully remove and drain on kitchen paper.
For the curry dressing, mix the curry paste and one tablespoon hot water together in a bowl to a smooth paste. Whisk in the vinegar, then slowly whisk in both the oils. Season, to taste, with salt and freshly ground black pepper.
To serve, arrange some watercress on serving plates and place the bhajis alongside. Drizzle over the curry dressing and top with a spoonful of yoghurt.