How to Make It
Heat oven to 400° F. Heat 1 tablespoon of the oil in a large ovenproof skillet over medium-high heat. Season the pork with ¼ teaspoon each salt and pepper and cook, turning occasionally, until browned, 6 to 8 minutes.
Transfer the skillet to oven and roast, turning once, until the internal temperature registers 145° F, 40 to 45 minutes. Remove the pork from the skillet and let rest 5 minutes before slicing. To the hot skillet, carefully add the vinegar, mustard, and parsley and stir to combine; set aside.
Meanwhile, in a second large skillet, heat the remaining tablespoon of oil over medium-high heat. Add the onion, zucchini and/or yellow squash, and ¼ teaspoon each salt and pepper and cook, tossing occasionally, until tender, 8 to 10 minutes. Serve with the pork and the mustard sauce.