Make the filling by placing the onion, ginger, garlic, chilli, and some salt into food processor. Purée until smooth. Heat the oil in a frying pan and pour the purée into it. Fry for 5 mins or until cooked through. Add the prawns, tomatoes and chopped coriander. Squeeze in the lime and add salt to taste. Cook for 3 mins, or until the prawns have cooked through. Remove from the heat.
Drain and rinse the black-eyed peas. Pour into a food processor with the garlic and chili. Purée until smooth. Scrape into a bowl and add the onion, flour, salt, chilli powder and baking powder. Mix and roll into 16 balls.
Heat the oven to 190C/170C fan/gas 5. Heat 8cm of the palm or vegetable oil in a wok or small heavy pan. When a small piece of bread sizzles, drop 4-5 balls into the oil. Fry until golden and crisp, about 4-5 mins. Drain on kitchen paper and repeat until they are all finished. You can keep them warm in the oven while you finish. Slice the fritters open down the centre and spoon the prawn filling in. Serve with hot sauce.