Preheat your oven to 375°F (190°C).
Cook the jumbo pasta shells in boiling salted water according to package directions until al dente, then drain and set aside.
In a skillet over medium heat, add the olive oil and ground turkey, seasoning with half of the salt, pepper, and garlic powder.
Cook the turkey until browned and crumbled, then stir in the marinara sauce and oregano; let simmer for 5 minutes.
In a small bowl, steam the spinach until wilted, squeeze out all excess moisture, and chop finely.
Mix the chopped spinach with the ricotta cheese and the remaining salt, pepper, and garlic powder until smooth.
Spread a thin layer of the turkey meat sauce in the bottom of a small baking dish.
Stuff each cooked shell generously with the ricotta-spinach mixture and place them on top of the sauce.
Top with the remaining sauce, shredded mozzarella, and parmesan cheese.
Cover with foil and bake for 15 minutes, then remove foil and bake for another 5-10 minutes until the cheese is melted and golden.