Pat the cecina dry with paper towels to ensure a good sear.
Heat the olive oil in a large skillet over medium-high heat until shimmering.
Add the cecina in a single layer (working in batches if needed) and cook 2–3 minutes per side, until browned and cooked through.
Transfer cooked cecina to a plate and keep warm.
Add the sliced onion to the skillet and cook 2–3 minutes, until softened and slightly caramelized.
Warm the tortillas in a dry skillet over medium heat, 30 seconds per side, or microwave wrapped in a damp paper towel until soft.
If desired, chop the cecina into smaller pieces for tacos.
Assemble by placing cecina on warm tortillas, topping with onions, a squeeze of lime juice, and chopped cilantro.
Serve with optional salsa, sliced avocado, and a sprinkle of flaky sea salt.