I recently made this, and it turned out to be the moistest pork tenderloin I have ever made! You can either bake it in the oven as noted, or grill outside.
Combine the lemon zest, lemon juice, olive oil, garlic, rosemary, thyme, mustard, and salt in a sturdy 1-gallon resealable plastic bag. Add the pork tenderloins and turn to coat with the marinade. Squeeze out the air and seal the bag. Marinate the pork in the refrigerator for at least 3 hours but preferably overnight.
Preheat the oven to 400 degrees F.
Remove the tenderloins from the marinade and discard the marinade but leave the herbs that cling to the meat. Sprinkle the tenderloins generously with salt and pepper. Heat 2 tablespoons olive oil in a large oven-proof saute pan over medium-high heat. Sear the pork tenderloins on all sides until golden brown.
Place the saute pan in the oven and roast the tenderloins for 10 to 15 minutes or until the meat registers 140 degrees F at the thickest part. Transfer the tenderloins to a platter and cover tightly with aluminum foil. Allow to rest for 10 minutes.
Carve in 1/2-inch-thick diagonal slices. Season with salt and pepper and serve warm, or at room temperature with the juices that collect in the platter.