Preheat your oven to 375°F (190°C).
Cook the jumbo pasta shells in a large pot of boiling salted water according to package directions until al dente, then drain and set aside.
In a medium non-stick skillet over medium heat, cook the ground turkey until browned and crumbled, about 5-7 minutes.
Roughly chop the fresh baby spinach and stir it into the turkey, cooking just until the leaves have wilted.
Remove the skillet from the heat and allow the turkey mixture to cool slightly.
In a medium mixing bowl, whisk the egg whites until combined, then fold in the ricotta cheese, turkey-spinach mixture, sea salt, black pepper, garlic powder, and dried oregano.
Spread half of the marinara sauce evenly across the bottom of a small baking dish.
Stuff each cooked pasta shell generously with the ricotta and turkey mixture, then arrange them in the baking dish.
Top the shells with the remaining marinara sauce and a sprinkle of grated parmesan cheese.
Cover the dish with foil and bake for 15 minutes, then remove the foil and bake for an additional 5 minutes until the sauce is bubbly.