2 1–1½-lb. pork tenderloins
3 tsp. Diamond Crystal or 2¼ tsp. Morton kosher salt, divided
2 tsp. freshly ground black pepper
2 Tbsp. thyme leaves
4 garlic cloves
¼ cup apple cider vinegar
¼ cup Dijon mustard
¼ cup honey
1 tsp. onion powder
½ tsp. red pepper flakes
2 Tbsp. neutral oil (such as grapeseed)
Season two 1–1½-lb. pork tenderloins all over with 2½ tsp. Diamond Crystal or 2 tsp. Morton kosher salt and 2 tsp. freshly ground black pepper and transfer to a large bowl or resealable bag. Add 2 Tbsp. thyme leaves and finely grate 4 garlic cloves over the pork.
Mix ¼ cup apple cider vinegar, ¼ cup Dijon mustard, ¼ cup honey, 1 tsp. onion powder, and ½ tsp. red pepper flakes in a small bowl until just combined. Season with remaining ½ tsp. Diamond Crystal or ¼ tsp. Morton 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 425°.
Heat 2 Tbsp. neutral oil (such as grapeseed) 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°, 14–18 minutes. Transfer pork to a cutting board and let rest at least 5 minutes before slicing ½” thick. Serve drizzled with reserved honey-mustard sauce.