Fill the saucepan with water and boil the potatoes for ten minutes or until tender. Drain the hot water out and let the potatoes cool down under cold running water. Rinse and dry with a kitchen towel.
Heat the oil in the frying pan and fry the onions for five minutes till they go tender and start getting golden. Add the garlic, chilli and crushed coriander and cumin seeds, and cook for 1 more minute while constantly stirring. Add the potatoes and fry everything together for another five minutes on high heat. Remove from heat, tip into a bowl and let to cool.
Peel the avocado and cut the flesh into chunks. Add to the potatoes with prawns and lime juice. Season with freshly ground pepper and toss gently.
In a small bowl, mix the yogurt and coriander and season with some pepper. In the four salad bowls, arrange the lettuce leaves. Spoon the salad into them and top with the coriander yogurt. Garnish with a lemon wedge. The salad is all ready to serve right away.