Bring a large pot of salted water to a boil. Cook jumbo shells for 9 minutes (1 to 2 minutes shy of package directions) so they stay firm and easy to handle. Drain and rinse gently with cool water.

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

In a large mixing bowl, combine ricotta, 1 cup of mozzarella, Parmesan, beaten egg, squeezed dry chopped spinach, minced garlic, Italian seasoning, salt, pepper, and nutmeg until well blended.

Spoon about 2 tablespoons of cheese filling into each cooked shell. Arrange filled shells open-side up in a single layer in the prepared baking dish.

Spoon remaining marinara sauce over and around the shells, then sprinkle with the remaining 1 cup of mozzarella cheese.

Cover dish loosely with aluminum foil (tented so cheese doesn't stick) and bake for 25 minutes. Remove foil and bake an additional 10 minutes until cheese is bubbly and golden brown on the edges.

Let rest for 5 minutes before serving. Garnish with fresh basil ribbons and extra grated Parmesan!