Place the prawns in a bowl. Add the turmeric, salt and fish sauce. Set aside to marinate for 10 minutes. In a mortar and pestle, pound the shallots, garlic, ginger and chilli to a fine paste.
Heat peanut oil in a wok or saucepan. Add the paste and turmeric powder. Sauté over low heat for 15 minutes until slightly caramelised. Add the chilli paste. Cook for a further 10 minutes.
Add the tomatoes and a dash of water. Cook for 10 minutes or until the sauce thickens.
Add the marinated prawns and cook for 2-3 minutes, turning once.
Garnish with the coriander to serve.