Marinate pork: Preheat oven to 450°F with a rack in the upper third. Add 1½ tablespoons of the teriyaki sauce to a medium bowl (save rest for step 2), then add pork and a pinch of salt; toss to coat pork.

Roast pork: Line a rimmed baking sheet with foil and transfer pork. Roast on upper oven rack until pork is firm to the touch and reaches an internal temperature of 145°F, about 15 minutes (watch closely as ovens vary). Remove from oven and brush with remaining teriyaki sauce. Set aside to rest for 5 minutes.

Make apple-celery slaw: Meanwhile, quarter apple, discarding core, then cut crosswise into matchsticks. Trim ends from celery, then thinly slice on an angle. In a medium bowl, whisk together 2 tablespoons oil, 1 tablespoon vinegar, and a generous pinch of sugar. Add apples and celery to dressing, then toss to coat. Season to taste with salt and pepper.

Make Sriracha-mayo: In a small bowl, stir together Sriracha and mayo. Season to taste with salt and pepper.

Char tortillas: Toast tortillas over an open flame until lightly charred in spots, 5–10 seconds per side. (Alternatively, bake on upper oven rack in batches until lightly charred, watching closely) Wrap in foil to keep warm.

Assemble tacos & serve: Coarsely chop cilantro leaves and tender stems together. Remove mint leaves from stems and coarsely chop leaves. Stir half of the cilantro and mint into apple-celery slaw. Thinly slice pork. Top tortillas with Sriracha-mayo, pork, some of the apple-celery slaw, and remaining herbs. Serve remaining apple-celery slaw alongside.