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.