Preheat the oven: Set the oven to 425 degrees F.
Boil the potatoes: Place sweet potatoes and salt in a medium saucepan with a lid. Add water to cover by 1-inch and put the lid on. Bring to a boil and let it simmer for 12-15 minutes or until potatoes are tender. Check doneness by inserting a knife into a potato. If it comes in and out easily, it is ready. Drain and return the potatoes to the pan.
Mash the potatoes: Add in the butter. Mash them using a potato masher (or a fork). Cook, stirring constantly, until all the water has evaporated, 3-4 minutes. It should slightly thicken. Taste for seasoning and add more if necessary. Set it aside.
Sauté the veggies: While the sweet potatoes are cooking, make the filling. Heat oil in a 12-inch cast iron skillet* over medium-high heat. Add onion and carrots and saute until they are softened, 8-10 minutes. Make sure that carrots are fully softened.
Cook the turkey: Add ground turkey and cook, breaking the large chunks with a wooden spoon, until it is cooked through, 10 minutes or so.
Add the herbs and spices: Stir in the garlic, fresh herbs (chopped thyme and rosemary), paprika, allspice, ground cumin, cinnamon, salt and pepper. Cook, stirring constantly, for one minute.
Add additional ingredients: Add tomato paste and chicken broth. Stir to combine. Let it simmer for 3-4 minutes or until some of the liquid has evaporated.
Stir in the peas: Stir in the frozen peas and cook for 1-2 more minutes. Give it a quick taste to make sure that it is seasoned well. If needed add more seasoning.
Top with potatoes: Evenly spread the mashed sweet potatoes over the filling, making sure that it is fully covered.
Bake: Bake for 15-17 minutes or until sweet potato topping is just beginning to brown. If preferred, brush it with a little bit of butter. Garnish with fresh herbs and serve.