For Mexican ceviche, first gut the squid and boil in salted water for 20 minutes until tender. Peel the prawns, remove the intestines and boil the prawns.
Peel and dice the potatoes and boil them in salted water until soft.
Peel, core and dice the tomatoes. Squeeze the juice from the limes. Cut the cooked seafood and the raw fish into cubes of about 1 cm and marinate with the lime juice. Leave to marinate for 15 minutes.
Drain off the excess liquid and mix the seafood and fish with the tomatoes, garlic, potatoes, shallots and chili. Season well with oil, salt and freshly ground pepper and let stand for 20 minutes.
Mix with the herbs just before serving. Peel the avocado, cut in half, remove the pit and cut the flesh into thin slices. Arrange the Mexican ceviche on the avocado slices and lettuce leaves.