In a large pot, bring salted water to a boil. Cook the jumbo pasta shells according to package instructions until al dente. Drain and set aside to cool.
In a skillet, heat the olive oil over medium heat. Add the minced garlic and sauté for about 1 minute until fragrant. Add the chopped spinach and cook until wilted. Remove from heat and let it cool slightly.
In a mixing bowl, combine the ricotta cheese, 1/2 cup of mozzarella cheese, 1/4 cup of Parmesan cheese, the sautéed spinach, onion powder, Italian seasoning, salt, and pepper. Mix until well combined.
Using a teaspoon or piping bag, fill each cooked pasta shell with a generous amount of the ricotta-spinach mixture.
Spread a small amount of marinara sauce on the bottom of a baking dish. Arrange the stuffed shells in the dish, seam side up. Pour the remaining marinara sauce over the top and sprinkle with the remaining mozzarella and Parmesan cheese.
Cover the baking dish with aluminum foil and bake in the preheated oven for 25 minutes. Remove the foil and bake for an additional 10 minutes until the cheese is bubbly and golden.
Allow to cool for a few minutes before serving. Garnish with fresh basil or parsley, if desired.
Notes
Garnish with fresh basil or parsley for added flavor.