- Sprinkle 1/4 teaspoon salt and 1/4 teaspoon pepper over both sides of the turkey.
- In a large skillet, heat the oil over medium-high heat, swirling to coat the bottom. Cook the turkey for 2 minutes on each side, or until browned. (The turkey won't be done at this point.) Transfer to a large plate.
- In the same skillet, stir together the sweet potatoes and onion. Reduce the heat to medium. Cook for 7 minutes, or until the onion is golden, stirring frequently.
- Stir in the apple, cinnamon, and the remaining 1/4 teaspoon salt and 1/4 teaspoon pepper. Cook for 5 minutes, stirring frequently.
- Place the turkey medallions among the sweet potato and apple slices. Pour in 2/3 cup cider. Bring to a boil, still over medium heat. Reduce the heat and simmer, covered, for 3 to 5 minutes, or until the turkey is no longer pink in the center and the sweet potatoes are tender. Remove from the heat. Transfer the turkey to a separate large plate, leaving the sweet potato mixture in the skillet. Cover the turkey loosely to keep warm.
- Using a potato masher, mash the sweet potato mixture. Stir in the walnuts. Transfer the sweet potato mixture to plates.
- Quickly pour the remaining 1/4 cup cider into the skillet, scraping to dislodge any browned bits. Stir in the mustard, green onions, and final pinch of pepper. Remove from the heat.
- Serve the turkey on the sweet potato mixture. Drizzle the sauce over the turkey.