Heat oven to 375°F (190°C). Boil shells 2 minutes less than the package al dente timing; they finish in the oven. Drain gently and spread on a lightly oiled tray so they do not stick together.

Put thawed spinach in a clean cloth and squeeze firmly until little liquid remains. Break it apart in a bowl. Excess water can make both the filling and sauce loose.

Stir ricotta, egg, Parmesan, 1 cup mozzarella, garlic, salt, pepper and optional nutmeg into the spinach until evenly combined. Do not taste the raw-egg filling; cook a small sample separately if adjusting seasoning.

Spread 1 1/2 cups marinara over the bottom of the baking dish. Spoon filling evenly among 24 shells, about 2 tablespoons each. Arrange opening side up. Spoon remaining sauce around and lightly over shells, then sprinkle with remaining 1/2 cup mozzarella.

Cover with foil, tenting it above the cheese. Bake 25 minutes, uncover and bake another 10–15 minutes until bubbling and the center filling reaches 165°F (74°C). If the top browns early, cover loosely and continue heating the center.

Rest for 10 minutes to let the filling settle. Scatter basil over the dish and serve four shells per portion.