Bring a large pot of salted water to a rolling boil. Add the jumbo pasta shells and cook according to package directions, typically 8-10 minutes, until just shy of al dente. They should still have a little bite, as they will finish cooking in the oven. Drain immediately and toss lightly with a drizzle of olive oil to prevent sticking. Set aside to cool slightly.

If using frozen spinach, thaw completely and place it in a cheesecloth or thick paper towels. Squeeze, squeeze, squeeze! You must remove as much water as possible. For fresh spinach, sauté it quickly in a tablespoon of olive oil until wilted, then squeeze out the moisture. Excess water will result in a runny ricotta cheese filling.

In a very large bowl, combine the ricotta cheese, 1 cup of grated mozzarella, the Parmesan/Pecorino, the beaten egg, minced garlic, Italian seasoning, salt, pepper, and that pinch of nutmeg. Add the super-dry, chopped spinach. Use a spoon or spatula to mix everything thoroughly until the spinach and ricotta mixture is perfectly uniform and the seasoning is well distributed.

Preheat your oven to 375°F (190°C). Lightly grease a large 9x13-inch baking dish. Pour about half of your marinara sauce into the bottom of the prepared dish, spreading it thinly to coat. This layer prevents the shells from sticking and keeps them moist while baking.

This is the fun, meditative part. Open up a cooked shell gently. Using a small teaspoon or a piping bag (if you’re feeling fancy), fill each shell generously with the creamy cheese mixture. Don’t overstuff them, but make sure they are plump and full. Arrange the filled shells seam-side up in the prepared baking dish. They should fit snugly, side-by-side, in a single layer.

Pour the remaining pasta sauce evenly over the stuffed shells. Ensure that most of the jumbo shells are well covered, except for the tiny mound of cheese peeking out. Finally, sprinkle the remaining 1.5 cups of shredded mozzarella and Parmesan topping over the sauced shells. You want a decent layer here for that classic, bubbly, golden-brown crust.

Cover the baking dish tightly with aluminum foil. Bake for 30 minutes. The foil ensures the shells heat through thoroughly without burning the cheese. After 30 minutes, remove the foil and continue baking for another 10-15 minutes, or until the cheese is melted, bubbly, and lightly golden brown. The sauce should be simmering vigorously around the edges.

Let the baked pasta casserole rest for 10-15 minutes on a wire rack before serving. This resting time helps the shells firm up, preventing the filling from spilling out when you plate them. Garnish with fresh herbs before bringing the magnificent dish to the table.