Bring a large pot of salted water to a rolling boil. Add jumbo pasta shells and cook for 8-9 minutes until just barely tender—they should still have slight firmness since they'll cook more in the oven. Drain carefully and spread on a clean kitchen towel to cool slightly.

While shells cook, combine thawed spinach, ricotta, 3/4 cup mozzarella, parmesan, egg, minced garlic, salt, pepper, nutmeg, and red pepper flakes in a medium bowl. Stir until fully incorporated and creamy. The mixture should hold together when spooned.

Preheat your oven to 375°F. Spread 1/2 cup marinara sauce across the bottom of a 9x13 baking dish. This prevents sticking and creates a flavorful base layer for your golden tender spinach stuffed shells recipe.

Fill each cooled shell with approximately 1.5 tablespoons of spinach mixture using a small spoon or piping bag. Arrange filled shells seam-side up in the prepared baking dish. Don't overcrowd—they should fit snugly but not overlap.

Pour remaining marinara sauce over the filled shells, ensuring each gets coated. Sprinkle remaining 1/4 cup mozzarella evenly across the top. Dot with butter to enhance browning.

Cover with foil and bake for 25 minutes. Remove foil and bake an additional 8-10 minutes until cheese is bubbly and edges are golden brown.

Let rest for 5 minutes before serving. This allows flavors to meld and makes plating cleaner.