Combine vinegar, sugar & salt. Add sliced onion, mix well, and set aside for at least 15 minutes.
Marinate Chicken
In a bowl, whisk together olive oil, lime zest and juice, garlic, cumin, paprika, chilli, salt, and pepper. Add tenderloins and coat evenly. Rest 10 minutes while making the sauce.
Coriander-Lime Yogurt
Blend yogurt, coriander, lime juice, and garlic until smooth. Season to taste.
Grill or Sear
Heat a grill pan or skillet over medium-high heat. Cook the chicken 2 minutes per side, until charred and cooked through (74°C / 165°F). Rest 2 minutes, then slice on a bias.
Assemble
Warm tortillas. Spread coriander-lime yogurt, top with sliced chicken, pickled onion, coriander leaves, and feta or cotija. Finish with a squeeze of lime.
Chef's Tips
Give the pickled onions the full 15 minutes, or longer if you have it — they need that time in the vinegar for the sharp raw-onion bite to mellow into something closer to tangy and sweet.Let the chicken sit in the marinade for the full 10 minutes, as written — with lime juice in there, longer isn't necessarily better since the acid can start to firm up the surface of the meat, but skipping the rest means the flavour doesn't get past the surface.Pull the chicken as soon as it hits 74°C (165°F) rather than going by time alone — tenderloins are thin and go from juicy to dry fast, so a thermometer check here is worth it over guessing.
Storage & Reheating
Store the chicken, coriander-lime cream, pickled onions and tortillas separately in the fridge rather than assembling ahead — the cream and pickled onion will make the tortilla soggy if added early.The chicken keeps in an airtight container for up to 4 days, or freeze for up to 3 months — it's a citrus marinade with no dairy in it, so it holds up well either way. Reheat gently in a pan or the microwave, taking care not to dry it out.The coriander-lime cream (yogurt-based) keeps refrigerated for up to 3 days — the coriander can lose its bright colour and flavour a bit beyond that, so it's best used earlier in the week rather than later.The pickled onions keep refrigerated for 2–3 weeks thanks to the vinegar. Assemble the tacos fresh with warm tortillas just before eating.