Place the peeled and quartered sweet potatoes in a large pot and cover with cold water. Bring to a boil, then reduce to a simmer. Cook for 15–20 minutes, or until fork-tender.
Drain the sweet potatoes and return them to the pot. Add 2 tablespoons olive oil and ½ teaspoon salt, then mash until smooth. Taste and adjust seasoning if needed.
Preheat the oven to 400°F (205°C).
Heat 1 tablespoon olive oil in a large oven-safe skillet over medium heat.
Add the diced onion and remaining ½ teaspoon salt. Cook for 5 minutes, until softened.
Stir in the ginger, garlic, and diced carrot. Cook for another 2–3 minutes.
Add the turmeric, cumin, coriander, cinnamon, and tomato paste. Stir well and cook for 1–2 minutes until fragrant. If needed, add another tablespoon of olive oil.
Pour in the vegetable stock and stir in the lentils. Bring to a boil, then reduce the heat and simmer uncovered for 10 minutes, until slightly thickened. Taste and adjust seasoning.
If your skillet is not oven-safe, transfer the filling to a 2-quart baking dish.
Spread the mashed sweet potatoes evenly over the lentil filling using a spatula or the back of a spoon.
Sprinkle the cumin seeds over the top.
Bake for 15–20 minutes, or until the sweet potato topping is lightly browned around the edges.
Let the shepherd’s pie rest for 5 minutes before serving. Garnish with chopped fresh cilantro if desired.