Place sweet potatoes in a medium saucepan, cover with lightly salted water and bring to a boil. Reduce heat to medium, cover and cook for 3 minutes. Add apple and cook until everything is just tender, but not mushy, 2 to 3 minutes longer. Drain.
Transfer 1 cup of the mixture to a large bowl; mash. Stir in sour cream and lemon juice. Add the remaining unmashed mixture and stir gently to mix. Set aside.
Heat oil in a large nonstick skillet over medium-high heat. Add onion and cook, stirring often, until softened, 2 to 3 minutes. Add turkey (or chicken), thyme, salt and pepper; cook, stirring occasionally, until heated through, about 2 minutes. dd the reserved sweet potato mixture to the skillet; stir to mix. Press on the hash with a wide metal spatula; cook until the bottom is lightly browned, about 3 minutes. Cut the hash into several rough sections; flip and cook until the undersides are browned, about 3 minutes longer. Serve immediately.