Start by finely dicing the onion and chopping the bell peppers. Mince the garlic cloves and set everything aside. Preheat your oven to 375°F (190°C) while you prepare the casserole.
In a large skillet or Dutch oven over medium heat, add the lean ground turkey. Cook until browned, breaking it apart with a spatula, about 5-7 minutes.
Add the diced onion and bell peppers to the skillet. Sauté for about 5 minutes until the vegetables are tender. Stir in the minced garlic and cook for an additional minute until fragrant.
Pour in the fire-roasted diced tomatoes (with juices) and marinara sauce. Sprinkle in the Italian seasoning, garlic powder, kosher salt, and ground black pepper. Stir to combine and let the mixture simmer for about 5 minutes.
Add the fresh baby spinach to the skillet, stirring until wilted. Next, stir in the quick cooking rice. Make sure everything is well mixed and heated through.
Transfer the turkey and vegetable mixture into a greased casserole dish, spreading it evenly.
Sprinkle the shredded mozzarella cheese generously over the top of the casserole.
Place the casserole in the preheated oven and bake for 25-30 minutes, or until the cheese is melted, bubbly, and slightly golden.
Once out of the oven, let it cool for a few minutes. Garnish with chopped fresh parsley before serving. Enjoy your Easy Stuffed Pepper Casserole with Ground Turkey!