Preheat your oven to 375°F and prepare a small baking dish with a light coating of olive oil.
Cook the jumbo pasta shells in a large pot of boiling salted water for 9 minutes until al dente, then drain and cool.
Brown the ground turkey in a skillet over medium heat until fully cooked through and set aside.
Finely chop the baby spinach and sauté briefly until wilted, then squeeze out all excess moisture using a clean kitchen towel.
In a medium bowl, stir together the ricotta cheese, egg whites, wilted spinach, cooked turkey, parmesan, garlic powder, oregano, salt, and pepper.
Spread half of the marinara sauce evenly across the bottom of the prepared baking dish.
Stuff each cooked shell generously with the ricotta and turkey mixture, then arrange them in the dish.
Top the shells with the remaining marinara sauce and sprinkle the shredded mozzarella cheese over the top.
Cover the dish with foil and bake for 20 minutes, then remove the foil and bake for 8 more minutes until the cheese is melted and the sauce is bubbling.