For the sauce
Sift the flour and bicarbonate of soda into a mixing bowl, add the spices and fresh coriander, and mix thoroughly. Divide the mixture between three separate small bowls. Stir the onion into one bowl, the leek into another and the cauliflower into the third. Add 4 tablespoons cold water to each bowl, and mix each one to form a smooth paste.
Heat enough oil for deep-frying in a wok or heavy pan. Using two tablespoons, form the mixture into rounds. Cooking in batches, fry the bhajias in the oil until golden brown. Remove with a slotted spoon and drain on kitchen paper.
Mix all the sauce ingredients together, and pour into a small serving bowl. Serve with the warm bhajias.