Heat a large, oven safe cast iron or saute pan over medium-high heat.
Remove leaves from two sprigs of rosemary and chop.
Coat the pork tenderloin liberally with salt, pepper, chopped rosemary, and 1 Tablespoon of olive oil.
Add 2 Tablespoons of olive oil to the hot pan, then add the onion around the edges.
Transfer the pork and sear for 3-5 minutes until deeply caramelized.
Turn the pork over, add the sliced persimmon to the pan, mixing with the onions.
Top with another tablespoon of olive oil, large pinch of salt, and remaining rosemary sprigs.
Transfer the pan to the oven and roast for 15-25 minutes depending on thickness, until cooked to your liking. I prefer a medium-well interior for pork tenderloin so I cook to 150-155 degrees F.