Place a large saucepan with water over high heat and cook the pasta al dente, according to package directions. Drain and set aside.
Make the cheesy filling
In a large skillet over medium-high heat, add the oil and when it starts to shimmer, add the garlic and cook until it begins to brown, about a minute.
Then, add the chopped spinach, stir and cook until it begins to wilt, about 3 to 4 minutes. Remove from the heat and let cool.
In a mixing bowl, stir together the cooked spinach, ricotta, mozzarella, Parmesan cheese, egg, onion powder, dried Italian herbs, dried parsley, dried basil, salt, and pepper until thoroughly combined.
Pour 1 cup of the marinara sauce into the bottom of a 9x13 baking dish.
Stuff each pasta shell with a hefty amount of the spinach and cheese mixture, and arrange in the baking dish.
Bake
Next, cover with the remaining marina sauce and cover with aluminum foil, bake for 25 minutes.
Remove the foil and continue baking until the top begins to brown, another 10-15 minutes.