Cook the potatoes in a saucepan of boiling water for 8 minutes or until just tender. Drain and rinse under cold running water. Dry in a clean tea-towel.
Heat the oil in a frying pan, add the onion and fry for 5 minutes or until softened and lightly browned. Add the garlic, chilli and crushed coriander and cumin seeds, and cook for 1 more minute, stirring. Stir in the potatoes and fry over a high heat for 3 minutes. Remove from the heat and leave to cool.
Peel the avocado, remove the stone and cut the flesh into small chunks. Add to the potatoes together with the prawns and lime juice. Season with pepper and toss gently.
Mix together the yogurt and coriander and season with pepper. Arrange 2 lettuce leaves in each of 4 bowls. Spoon the salad into them and top with the coriander yogurt.