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 non-stick skillet over medium heat, cook the ground turkey until browned and crumbled, seasoning with half of the oregano, salt, and pepper.
Add the fresh baby spinach to the skillet with the turkey and cook for 1-2 minutes until just wilted, then remove from heat.
In a small mixing bowl, combine the part-skim ricotta, liquid egg whites, garlic powder, and the remaining oregano, salt, and pepper.
Stir the wilted spinach into the ricotta mixture until well combined.
Spread half of the marinara sauce on the bottom of a small baking dish.
Stuff each cooked pasta shell generously with the spinach-ricotta mixture and place them in the baking dish.
Top the shells with the cooked ground turkey and the remaining marinara sauce.
Sprinkle the shredded parmesan cheese over the top.
Cover with foil and bake for 15 minutes, then remove the foil and bake for an additional 5-10 minutes until the sauce is bubbling and the cheese is golden.