1. Peel the cucumber and cut crossways into 1cm (½in) thick rounds. Peel the shallots and finely chop three quarters of them; finely slice the rest. Peel the garlic cloves. Chop four of them very finely; cut the other two into fine slivers. Combine the sliced shallots and slivered garlic and set these aside.
2. In a medium pan, combine the chopped shallots, chopped garlic, ground coriander seeds, ground fennel seeds, white pepper, cumin seeds, turmeric and 450ml (15fl oz) water. Crumble in the red chillies. Stir and bring to the boil. Boil, uncovered, on a fairly high heat for about 5 minutes.
3. Add the cucumber rounds and bring to a simmer. Cover and simmer gently for 5 minutes. Add the prawns, salt and sugar. Bring to a simmer again and simmer gently for about 1 minute, stirring the prawns around in the sauce. Give the coconut milk a good stir and pour it in. Bring the mixture to the boil, then lower the heat and simmer for 1 minute, stirring now and then.
4. Put the oil in a small frying pan and set over a medium–high heat. When hot, add the slivered shallots and garlic and stir-fry until they turn golden. Add the whole fennel seeds, stir once, then quickly pour the contents of the pan (oil and seasonings) into the pan containing the curry. Cover the curry pan immediately to trap all the aromas. Serve with rice.