Preheat the oven to 375°F and spray a shallow 8x8 inch baking dish with nonstick spray; set aside.
Bring a pot of water on the stovetop to a boil. Cook the pasta shells to al dente. After cooking the pasta shells, drain and place them upside down on a baking sheet to dry.
16 jumbo pasta shells
Heat the olive oil in a large skillet over medium-high heat. Add the garlic and cook until fragrant, about a minute or two.
1 tablespoon olive oil, 3 garlic cloves
Add the spinach and cook, stirring occasionally, for about 4 minutes until the leaves start to wilt. Remove from the heat and let the spinach cool.
4 cups lightly packed baby spinach leaves
In a large mixing bowl, stir together the spinach, ricotta, mozzarella, parmesan, egg, salt, and pepper until thoroughly combined.
1½ cups ricotta cheese, 1 cup shredded mozzarella cheese, ¾ cups grated parmesan cheese, 1 egg, 1 teaspoon salt, ½ teaspoon freshly ground black pepper
Pour half of the marinara sauce into the bottom the baking dish.
1½ cups marinara sauce
Working 1 shell at a time, generously fill each pasta shell with the spinach and ricotta mixture. Carefully place each shell in the baking dish, filling side up.
Once all shells are filled and in the baking dish, cover with the remaining marinara sauce and a sprinkle of shredded mozzarella cheese.
Cover the baking dish with aluminum foil and bake for 20 minutes.
Remove the aluminum foil and bake for an additional 15 minutes, until the cheese starts to bubble and begins to brown.
Serve with a sprinkle of parmesan cheese and fresh basil leaves.
Fresh basil leaves for serving