If your mussels are fresh: wash well and place in a saucepan with a splash of white wine or water. Cover with a lid and let steam for 3-5 minutes, until they are open.
If your mussels are frozen: cook according to instructions in the packet.
Let mussels cool down.
Boil corn kernels for 5 minutes, let cool down.
Mix onion, tomato, chilli and corn. Season with lime juice and salt. Add coriander, taste and adjust seasoning if needed.
Split mussel shells in half and discard empty half. Make sure mussels are detached from the shells. Arrange on a plate or platter and spoon salsa chalaca on top.
Serve with a bowl of cancha on the side if you can find it.