Bring a large pot of well-salted water to a rolling boil. Cook jumbo pasta shells for 8-10 minutes until just al dente. Drain and lay out on lightly oiled foil or parchment to prevent sticking.

Heat olive oil in a skillet over medium heat. Add minced garlic and cook for 30 seconds until fragrant. Add chopped spinach and cook for 2-3 minutes until wilted. Remove from heat and cool slightly.

In a large mixing bowl, combine ricotta, 1/2 cup mozzarella, Parmesan, and egg. Stir until smooth. Add cooked spinach and garlic. Season with Italian seasoning, salt, pepper, and optional pinch of nutmeg. Taste and adjust seasoning.

Preheat oven to 375F (190C). Spread 1 cup marinara sauce evenly across the bottom of a 9x13 inch baking dish.

Fill each shell with about 2 tablespoons of the ricotta-spinach mixture using a spoon or piping bag. Do not overstuff. Arrange open side up in a single layer in the baking dish.

Spoon the remaining 1 cup marinara over the shells, covering each one well. Sprinkle the remaining 1/2 cup mozzarella evenly on top. Cover tightly with aluminum foil and bake for 20 minutes.

Remove foil and bake for an additional 10 minutes until cheese is melted, golden in spots, and bubbling at the edges. Rest for 5 minutes before serving. Garnish with fresh basil if desired.