Preheat your oven to 375°F (190°C).
Cook the jumbo pasta shells in a large pot of boiling water until al dente, then drain and set aside to cool slightly.
Steam the fresh spinach until fully wilted, then place it in a clean kitchen towel and squeeze out as much moisture as possible before chopping finely.
In a medium mixing bowl, stir together the ricotta cheese, chopped spinach, liquid egg whites, sea salt, black pepper, garlic powder, and dried oregano until well combined.
Spread roughly half of the marinara sauce across the bottom of a small oven-safe baking dish.
Generously stuff each cooked pasta shell with the spinach and ricotta mixture, then arrange them in the baking dish.
Spoon the remaining marinara sauce over the shells and sprinkle evenly with the mozzarella and parmesan cheeses.
Cover the dish with foil and bake for 20 minutes; remove the foil and bake for an additional 5 to 10 minutes until the cheese is melted and bubbling.