Prepare the Sweet Potatoes:

1. Place the cubed sweet potatoes in a large pot and cover with cold water. Add a pinch of salt and bring to a boil over high heat.
2. Reduce heat to medium-high and cook for 15-20 minutes, until the sweet potatoes are fork-tender and easily pierced.
3. Drain the sweet potatoes thoroughly and return them to the pot. Add butter, milk, salt, pepper, and nutmeg if using.
4. Mash until smooth and creamy, adjusting milk as needed for your preferred consistency. Set aside.

Make the Meat Filling:

1. Preheat your oven to 400°F (200°C).
2. Heat olive oil in a large, oven-safe skillet or Dutch oven over medium-high heat. Add the diced onion, carrots, and celery.
3. Cook for 5-7 minutes, stirring occasionally, until the vegetables begin to soften.
4. Add the minced garlic and cook for another minute until fragrant.
5. Add the ground lamb (or beef) to the pan, breaking it up with a wooden spoon. Cook for 8-10 minutes until the meat is browned and cooked through.
6. Stir in the tomato paste, Worcestershire sauce, thyme, and rosemary. Cook for 2 minutes to let the flavors blend.
7. Sprinkle the flour over the meat mixture and stir to combine. Cook for 1 minute.
8. Gradually add the broth, stirring constantly to prevent lumps. Add the frozen peas and simmer for 3-4 minutes until the mixture thickens slightly.
9. Season with salt and pepper to taste.

Assemble and Bake:

1. If your skillet isn’t oven-safe, transfer the meat mixture to a 9×13 inch baking dish.
2. Spread the sweet potato mash evenly over the top of the meat filling, using a fork to create some texture on the surface.
3. Bake for 25-30 minutes, until the sweet potato topping is lightly golden and the filling is bubbling around the edges.
4. Let the shepherd’s pie rest for 5-10 minutes before serving to allow it to set up slightly.