Season two 1–1½-lb. pork tenderloins all over with 2 tsp kosher salt and 2 tsp. freshly ground black pepper and transfer to a large bowl or resealable bag. Add the thyme leaves and finely grate or crush 4 garlic cloves over the pork.

Mix 1/4 cup apple cider vinegar, 1/4 cup Dijon mustard, 1/4 cup honey, 1 tsp. onion powder, and red pepper flakes in a small bowl until just combined. Season with remaining ½ tsp. kosher salt. Pour half of honey-mustard sauce over pork and turn to coat. Reserve remaining honey-mustard sauce for serving. Cover or seal bag and let marinate, refrigerated, for at least 1 hour and up to overnight.

Preheat oven to 425F/220C.

Heat 2 tbs oil in a large cast-iron skillet over medium-high. Remove pork from marinade and let excess drip off. Sear pork, turning occasionally, until browned on all sides, 6–8 minutes.

Transfer skillet to oven and roast, flipping pork halfway through, until an instant-read thermometer inserted into the thickest part registers 140°F/70C 14–18 minutes. Transfer pork to a cutting board and let rest at least 5 minutes before slicing 1/2-inch thick. Serve drizzled with reserved honey-mustard sauce.

For air fryer--browning in the skillet is optional but it does produce a better color. Air fry at 400F/200C for 18-20 minutes. Test with instant read thermometer.