1. If making a quick curtido, place the thinly sliced red onion in a bowl, sprinkle with a pinch of salt, and squeeze over some lime juice. Toss and let sit while you prepare the salad so it softens and mellows.
2. In a large bowl, add the drained tuna and gently break it up with a fork into chunky flakes.
3. Add the cubed cooked yuca to the bowl with the tuna. Drizzle in the mayonnaise and the juice of 2 limes. Gently fold everything together so the yuca stays mostly intact.
4. Stir in the chopped cilantro and 1–2 tablespoons of tomate de árbol ají. Taste and add more lime, ají, or salt as needed. The flavor should be bright, a little spicy, and well seasoned.
5. If using the onion curtido, drain off any excess liquid and fold some into the salad, or pile it over the top for a more encebollado-style presentation. If skipping curtido, add the raw thinly sliced onion directly into the salad and mix gently.
6. Right before serving, top the salad with the diced or sliced avocado. Finish with an extra squeeze of lime and a pinch of salt over the avocado.
7. Serve the ensalada de yuca y atún on a shallow platter, crowned with avocado and a generous handful of plantain chips. Offer extra plantain chips and more tomate de árbol ají on the side so everyone can add crunch and heat at the table.