Preheat your oven to 375°F (190°C).
Heat the olive oil in a large oven-safe skillet over medium-high heat. Add the ground lamb and cook, breaking it up with a spoon, until browned. Drain off any excess fat.
Add the chopped onion, diced carrots, and diced celery to the skillet. Cook, stirring occasionally, until the vegetables have softened, about 8-10 minutes. Stir in the minced garlic and cook for another minute until fragrant.
Sprinkle the flour over the meat and vegetable mixture. Stir well and cook for 1 minute to coat everything.
Pour in the beef broth and red wine (if using). Stir in the Worcestershire sauce, dried thyme, dried rosemary, salt, and pepper. Bring the mixture to a simmer, then reduce the heat to low and let it cook for 10-15 minutes, stirring occasionally, until the sauce has thickened.
While the filling simmers, place the sweet potato chunks in a medium pot and cover with cold water. Bring to a boil and cook for 15-20 minutes, or until fork-tender. Drain the sweet potatoes thoroughly.
Return the drained sweet potatoes to the pot. Add the milk and butter. Mash until smooth and creamy. Stir in the fresh parsley, chopped chives, salt, and pepper to taste.
Spread the sweet potato mash evenly over the lamb filling in the skillet. You can create decorative swirls with a fork if desired.
Place the skillet in the preheated oven and bake for 20-25 minutes, or until the topping is lightly golden and the filling is bubbling around the edges.
Let the Shepherd's Pie rest for 5-10 minutes before serving.