Heat the oven to 400°F.

• Cut the sweet potatoes into 1/2-inch-thick slices

In a mixing bowl, toss the sweet potatoes with 1 tablespoon oil, salt, and pepper. On a sheet pan, spread the sweet potatoes onto a sheet pan in an even layer and roast until well browned and cooked through, turning once halfway through the cooking time, 20 to 25 minutes. While the potatoes cook, prep the remaining ingredients.

• Season the pork with salt and pepper.
• Cut the onions into thin slices.
• Cut the apples into ½-inch-thick dice.
• Strip the rosemary leaves from their stems.
• Coarsely chop the parsley leaves.

In a frying pan over medium-high heat warm 1 to 2 teaspoons oil until hot but not smoking. Add the pork and cook until well browned and barely pink in the middle, 3 to 4 minutes per side. Transfer the pork to a plate to rest.

In the same pan used to brown the pork, warm 1 tablespoon oil or butter (if using) until hot but not smoking. Add the onions, season with salt, and cook, stirring occasionally, until the onions are starting to soften and caramelize, 2 to 3 minutes. Add the apples and rosemary and cook, stirring occasionally, until the apples begin to soften, 3 to 4 minutes.
Add the vinegar and chicken stock to the pan. Bring the liquid to a boil, reduce the heat to medium and return the pork to the pan until is just heated through and the liquid has reduced by about half.

Transfer the apple-onion mixture to individual plates. Top with the pork lions and serve with the sweet potatoes on the side.