01 - Preheat oven to 400°F. Lightly grease a 9x13-inch baking dish with cooking spray or a small amount of oil.
02 - Heat olive oil in a large skillet over medium heat. Add diced onion and red bell pepper; cook for 3-4 minutes until softened and translucent.
03 - Stir in minced garlic and cook for 1 minute until fragrant, being careful not to burn it.
04 - Add ground turkey to the skillet, breaking it up with a wooden spoon. Cook for 5-6 minutes until completely browned and no longer pink.
05 - Sprinkle smoked paprika, dried thyme, ground cumin, chili powder, salt, black pepper, and red pepper flakes over the turkey. Stir thoroughly to distribute spices evenly.
06 - Add chopped spinach to the skillet and cook for 1-2 minutes, stirring until just wilted. Remove skillet from heat.
07 - Place sweet potato cubes in a large bowl and season with a pinch of salt and pepper. Toss to coat, then spread evenly in the prepared baking dish.
08 - Spoon the turkey and vegetable mixture evenly over the sweet potato layer. Drizzle milk over the entire surface, then sprinkle shredded cheddar cheese uniformly across the top.
09 - Cover the baking dish tightly with aluminum foil. Bake for 25 minutes to allow sweet potatoes to begin softening.
10 - Remove foil and return to oven for 15 minutes until sweet potatoes are tender when pierced with a fork and cheese is golden brown and bubbly.
11 - Let the casserole rest for 5 minutes before serving. Garnish with fresh chopped parsley if desired. Serve warm.