Preheat your oven to 400°F (200°C). While the oven is warming up, prepare your baking dish by lightly greasing it with a small amount of oil or non-stick spray.
Place the cubed sweet potatoes in a large pot of salted water. Bring to a boil and cook for about 10 minutes, or until the cubes are fork-tender. Drain the water and set the potatoes aside.
In a large skillet, heat the olive oil over medium heat. Add the diced onion and bell pepper, sautéing until they begin to soften. Stir in the minced garlic for about one minute until fragrant. Add the ground turkey, smoked paprika, cumin, salt, and pepper. Cook the turkey until it is browned and cooked through, breaking it apart with a spatula as it cooks.
Take half of your boiled sweet potato cubes and mash them in a bowl. Mix the mashed potatoes into the cooked turkey mixture until well combined. This creates a creamy texture without the need for added milk or cream.
Spread the turkey and mashed potato mixture evenly into your prepared baking dish. Top the mixture with the remaining whole sweet potato cubes. Sprinkle the shredded mozzarella cheese over the top if you are using it. Place the dish in the oven and bake for 15-20 minutes, or until the cheese is bubbly and starting to turn golden brown.
Remove the casserole from the oven and let it rest for 5 minutes. Garnish with fresh parsley before serving to add a pop of color and freshness.