Marinate the pork tenderloin in the Worcestershire sauce, olive oil, garlic, shallot, black pepper and thyme, turning half way through. (For at least 1 hour, up to 4 hours)
Preheat the oven to 375F.
Heat oil in a large sauté pan over high heat. Season pork with a small amount of salt & pepper and reserve the marinade.
Sear the pork tenderloin on all sides, then transfer to a baking sheet. Bake until internal temperature reaches 155F.
Meanwhile, add the potatoes and parsnips to the pan, add the reserved marinade and turn lower heat to medium.
Continue sautéing the potatoes and parsnips until they are almost cooked through, then add the pears.
Add the cider vinegar and brown sugar. Continue to sauté until vegetables are soft and sauce is reduced. NOTE: If you feel like your sauce is getting too thick or reducing too fast, just add a little water to the pan.
After pork has rested, slice and serve over pears, parsnips and potatoes. Top with the reduced sauce and parsley to serve.