In a medium bowl, combine onion, 45ml (3 Tbsp) lime juice, cilantro and 1ml (1/4 tsp) salt; let stand until wilted, tossing occasionally, about 30 minutes.
Meanwhile, in a zip-top food storage bag (or a glass container with lid), combine remaining 45ml (3 Tbsp) lime juice, remaining 2ml (1/2 tsp) salt, oil, garlic, oregano, cumin and pepper. Add pork to bag (or container); seal bag and turn to coat pork with mixture (or toss in container and cover). Refrigerate at least 30 minutes or up to several hours.
Off heat, coat a grill rack or stove-top grill pan with cooking spray; heat grill to medium-hot or grill pan over medium-high heat.
Remove pork from marinade; discard marinade. Grill pork, flipping once, until an instant-read thermometer inserted in centre of a pork chop registers 63°C (145°F) (for medium), about 8 minutes total.
Transfer to serving plates and let pork sit for 3 minutes; sprinkle with marinated onions.
Yields 1 pork chop and about 60ml (1/4 cup) onions per serving.