Prepare the marinade by whisking together the whole grain mustard, maple syrup, Dijon mustard, olive oil, minced garlic, chopped herbs, salt, pepper, and optional cayenne in a bowl until well combined.
Pat the pork tenderloins dry thoroughly with paper towels. This crucial step ensures proper searing and caramelization.
Coat the pork evenly with the maple-mustard mixture, using your hands to massage it into all surfaces. For deeper flavor, place in a sealed container or zip-top bag and refrigerate for 2-8 hours, though you can cook immediately if pressed for time.
Preheat your oven to 375°F (190°C) and position a rack in the middle of the oven.
Heat a large oven-safe skillet over medium-high heat until very hot. Add the neutral oil and swirl to coat the pan.
Sear the tenderloins for 2-3 minutes on each side until deeply golden brown, being careful not to burn the sugars in the marinade. Work in batches if necessary to avoid crowding the pan.
Transfer the skillet to the preheated oven and roast for 15-20 minutes until the internal temperature reaches 145°F (63°C) for medium.
Remove from oven and tent loosely with aluminum foil. Let the meat rest for 10 minutes to allow juices to redistribute throughout the pork.
Slice into medallions against the grain, about ½-inch thick, and arrange on a serving platter.
Drizzle any remaining pan juices over the sliced pork just before serving for an extra flavor boost.