Pat the cecina dry with kitchen paper to ensure it browns well.
Heat the olive oil in a large frying pan over medium–high heat until it shimmers.
Add the cecina in a single layer (you may need to work in batches) and cook for 2–3 minutes on each side, until browned and cooked through.
Transfer the cooked cecina to a plate and keep warm.
Add the sliced onion to the pan and cook for 2–3 minutes, until softened and lightly caramelised.
Warm the tortillas in a dry frying pan over medium heat for about 30 seconds on each side, or wrap them in a damp tea towel and microwave until soft.
If you like, chop the cecina into smaller pieces to make tacos.
To assemble, place the cecina on the warm tortillas, top with the onions, a squeeze of lime juice and the chopped coriander.
Serve with salsa and sliced avocado, and finish with a sprinkle of flaky sea salt.