Preheat the oven to 180°C (356°F). Season the chicken breast with salt and pepper.
Heat a frying pan over high heat with a little neutral oil and sear the chicken 1–2 minutes per side until browned.
Transfer the seared chicken to an ovenproof dish, cover tightly with aluminum foil, and bake for 25–30 minutes until cooked through.
Meanwhile, make the tomatillo salsa: add the pickled green tomatoes, diced half white onion, and pickled jalapeños to a blender or food processor and pulse until chunky.
When the chicken is done, shred it with two forks. Heat a drizzle of neutral oil in a frying pan over high heat.
Add the shredded chicken to the pan, sprinkle with Cajun seasoning, pour in the barbecue sauce, and fry 1–2 minutes, tossing to coat and heat through.
Warm the tortillas in a dry pan or oven, then assemble: divide the pulled chicken among the tortillas, spoon over tomatillo salsa, add a little sour cream, and finish with crispy fried onions.