1. Make the pork marinade by combining the soy sauce, garlic powder, cayenne pepper and cilantro. Whisk until mixed. Place the pork in a large Ziploc bag then pour over the marinade. Place in the fridge for an hour or up to 24 hours.
2. In a large bowl, combine the zucchini and red onion plus the soy sauce and black pepper.
3. Heat a cast iron grill pan on medium high until just smoking. Carefully brush on some extra virgin olive oil then throw on the zucchini. Cook it for 4 minutes per side then set aside.
4. Repeat for the red onion, cooking this time only 2 minutes per side then set aside with the zucchini.
5. Lower the heat to medium and throw on the pork tenderloin. Cook for 2 minutes on all sides to get a nice crusty sear. Continue to cook on medium, rotating the pork until it reaches an internal temperature of 145 degrees, it should take about 20 minutes. Remove from the grill pan and let rest for 10 minutes with foil covering it.
6. Serve with the grilled vegetables for an easy dinner.