Combine pork medallions with 1/2 cup apple cider, garlic, thyme, salt, and pepper in a bowl; marinate for 15-30 minutes or up to 2 hours in the refrigerator.
Boil salted water in a large pot; cook cubed sweet potatoes for 15-20 minutes until tender, then drain and return to pot.
Heat olive oil in a large skillet over medium-high heat; remove pork from marinade and sear for 3-4 minutes per side until browned, then set aside.
Sauté sliced onion in the same skillet for 3-4 minutes until softened.
Add reserved marinade and remaining 1/2 cup apple cider to the skillet; bring to a simmer.
Return pork to skillet; cover and cook 10-15 minutes until pork reaches 145°F internal temperature.
Optional: Mix cornstarch with water to make slurry; stir into sauce and cook 2-3 minutes until thickened.
Stir in 1 tablespoon butter into sauce for richness and flavor.
Add butter, heavy cream or milk, salt, and cinnamon to sweet potatoes; mash until smooth and creamy.
Serve mashed sweet potatoes topped with apple cider pork and drizzle with sauce; garnish with fresh thyme if desired.