To make the topping: Peel the sweetpotatoes, and cut them into rough 2 inch chunks. Place the potatoes in a medium saucepan, cover with water and bring to a boil. Reduce to a simmer and cook until the potatoes are very tender.
Drain the potatoes and return them to the pan. Place the pan over low heat and add the butter, brown sugar and salt. Using a fork, or device of your choice, mash the potatoes until smooth and the butter and sugar are fully incorporated. Set aside.
To make the filling: In a 12-inch saute pan or dutch oven, heat 1 tablespoon of olive oil over medium heat. Add the onion and 1 teaspoon of salt. Sweat for about five minutes, or until the onions are translucent. Push the onions to the edge of the pan and force them into a tight mound.
Add the remaining olive oil and the mushrooms. Allow them to sit and caramelize in the hot pan for about one minute. Resist the urge to shake the mushrooms around as you are trying to develop flavor by letting them brown. After two minutes, stir together the onions and the mushrooms, making sure you scrape up any browned bits from the bottom of the pan. Remove from saucepan and set aside.
Using the same saucepan, add the final tablespoon of olive oil. Add the turkey, and begin breaking it up with a spatula. Season the turkey with the remaining salt, garlic, chipotle powder, paprika, turmeric and cumin. Allow the turkey to brown and the spices to toast, about 4 minutes.
Stir in the onion mixture, tomato sauce, spinach and water. Let this all cook down until it’s like a very thick and saucy. Remove from the heat.
Assemble and Bake: Preheat your oven to 350° Spread the turkey mixture onto the bottom of a 10×12 inch baking dish. Spoon the sweetpotato over the top so that it’s in a even layer. Sprinkle the sweetpotato with the cheese.
Bake for 1 hour uncovered. Allow it to cool for about 10 minutes before serving.