Preheat your oven to 375 degrees F. Bring a large pot of water to a boil, add 1 tablespoon of kosher salt, and cook the jumbo shells for 2 minutes less than the package directions say. You want them pliable enough to fill but firm enough that they hold their shape. They will finish cooking in the oven. Drain them carefully, rinse with cold water so they stop cooking, and lay them out on a clean kitchen towel or parchment so they do not stick together.

Squeeze every bit of water out of the thawed spinach. This step is not optional. Wet spinach will make the filling watery and soggy. Wrap it in a clean kitchen towel and wring it hard over the sink until almost no liquid comes out.

In a large mixing bowl, stir together the ricotta, cottage cheese, squeezed spinach, beaten egg, minced garlic, 1/2 teaspoon salt, black pepper, nutmeg, and dried oregano until everything is evenly combined. Stir in 1 cup of the shredded mozzarella and 3 tablespoons of the Parmesan. Taste the filling and adjust the salt if needed.

Spread 1 cup of marinara sauce across the bottom of a 9x13 inch baking dish in an even layer. This keeps the shells from sticking to the bottom.

Use a small spoon to fill each pasta shell with about 2 heaping tablespoons of the cheese and spinach filling. Nestle the filled shells side by side in the baking dish, open side up. Do not pack them too tightly.

Spoon the remaining marinara sauce evenly over and around the stuffed shells. Sprinkle the remaining 1/2 cup of mozzarella and the remaining Parmesan over the top.

Cover the baking dish tightly with aluminum foil. Bake for 20 minutes covered, then carefully remove the foil and bake for another 12 to 15 minutes. The dish is ready when the sauce is bubbling at the edges and the cheese on top is melted and lightly golden in spots. Let it rest for 5 minutes before serving so the filling sets up slightly.