Cook shells until al dente, drain, and cool so you can handle them. Lay them flat on a sheet.
Mix ricotta, 1 cup mozzarella, Parmesan, spinach, egg, garlic powder, salt, and pepper in a bowl until combined.
Spoon about 2 tablespoons filling into each shell and place in a 9x13 dish with 1 cup sauce spread on the bottom.
Cover shells with remaining sauce, sprinkle 1/2 cup mozzarella on top, and cover tightly with foil.
Freeze flat for up to 3 months, or bake at 375°F for 25–30 minutes covered, then 5–10 minutes uncovered until bubbly.
Prep time: 20 minutes | Cook time: 30 minutes | Servings: 6