How to Make It
Preheat oven to 400°F. Sprinkle pork with 3/4 teaspoon each of the salt and pepper. Add 1 tablespoon of the oil to a large ovenproof skillet over medium-high. Add pork to skillet; cook until well browned, about 8 minutes, flipping once halfway through cook time. Remove pork to a plate, reserving drippings in skillet. Add 1 tablespoon of the oil to skillet. Add potatoes; cook over medium-high, undisturbed, until starting to brown, about 3 minutes. Stir in remaining 3/4 teaspoon salt and 1/4 teaspoon pepper. Return pork to center of skillet; arrange potatoes around pork.
Transfer skillet to preheated oven; roast 12 minutes. Remove from oven; stir peas into potatoes. Return to oven; roast until a thermometer inserted in thickest portion of tenderloin registers 145°F, about 5 minutes.
Remove skillet from oven. Transfer pork to a cutting board; transfer peas and potatoes to a serving platter. (Reserve drippings in skillet.) Allow pork to rest 5 minutes.
Add vinegar, mustard, and remaining 1 tablspoon oil to skillet; whisk to combine. Cut pork crosswise into 16 slices. Arrange pork alongside peas and potatoes on a platter. Drizzle with mixture from skillet, and sprinkle with parsley.