Preheat the oven to 375°F (190°C) and place a rack in the top third of the oven.
Bring a large pot of salted water to a boil and cook the jumbo shells 1 minute less than the package directions. Drain and transfer to a colander to cool slightly so they are easy to handle.
Spread the marinara sauce evenly in the bottom of a 9×13-inch baking dish.
Thoroughly squeeze excess water from the thawed spinach and chop if needed.
In a mixing bowl combine the drained spinach, ricotta, grated Parmesan, egg, salt, black pepper, Italian seasoning, and garlic powder until well mixed.
Spoon the filling into each cooked shell using a small spoon, then place the filled shells seam-side up in the prepared baking dish on top of the marinara sauce.
Sprinkle the shredded mozzarella evenly over the stuffed shells and cover the dish tightly with foil.
Bake covered for 30 minutes, then remove the foil and broil or bake a few additional minutes until the cheese is golden and bubbly—watch carefully to avoid burning.
Serve immediately, seasoning with extra salt and pepper if desired and garnish with fresh basil or parsley.
