1. Place sweet potatoes in a medium saucepan, cover with 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 (2-3 minutes longer). Drain off liquid.
2. Transfer 1 cup of the mixture to a large bowl and mash. Stir in sour cream and lemon juice. Gently stir in the remaining potatoes and apples.
3. Heat oil in a large nonstick skillet over medium-high heat. Add onion and cook, until softened (2-3 minutes). Add turkey, thyme, salt, and pepper. Cook, stirring until heated through (2-3 minutes).
4. Fold in the sweet potato mixture. Cook without stirring until the bottom is lightly browned (3-4 minutes). Cut the hash into several rough sections; flip and cook until the undersides are browned (3-4 minutes).