Preheat the oven to 375°F (190°C). Lightly grease a 9x13-inch baking dish and set aside.
Bring a large pot of salted water to a boil and cook the jumbo pasta shells according to package directions until al dente. Drain and set aside to cool so they are easy to handle.
Heat the olive oil in a large skillet over medium-high heat. Add the minced garlic and cook until fragrant and beginning to brown, about 1 minute.
Add the chopped spinach to the skillet and cook, stirring, until wilted, about 3–4 minutes. Remove from heat and let cool slightly, then squeeze out excess liquid from the spinach.
In a mixing bowl combine the drained spinach, ricotta, shredded mozzarella, grated Parmesan, beaten eggs, onion powder, dried Italian herbs, dried parsley, dried basil, kosher salt, and black pepper. Stir until evenly combined.
Spread 1 cup of marinara sauce across the bottom of the prepared baking dish.
Stuff each cooked shell with a generous portion of the spinach-ricotta mixture and arrange the filled shells in the baking dish in a single layer.
Pour the remaining marinara sauce evenly over the arranged shells and cover the dish tightly with aluminum foil.
Bake covered for 25 minutes. Remove the foil and bake uncovered for an additional 10–15 minutes, until the top is bubbly and beginning to brown.
Let the dish rest a few minutes, then serve warm topped with extra grated Parmesan and fresh basil.
