Preheat the oven to 375°F.

Bring a large pot of heavily salted water to a boil. Cook the shells 1–2 minutes shy of al dente.

To cool them quickly and stop carryover cooking, toss a couple of ice cubes into the colander while draining. Transfer the shells to a parchment-lined baking sheet in a single layer so they don’t stick and allow them to finish cooling.

In a large bowl, combine the ricotta, spinach, mozzarella, Parmigiano Reggiano, egg, and basil.

Season with salt and black pepper and mix until smooth and evenly combined.

Spread a thin layer of tomato sauce across the bottom of a 9×13 baking dish.

Fill each cooled shell with about 1–2 tablespoons of the ricotta mixture and arrange them open-side up in the dish.

Spoon the remaining sauce evenly over the shells and finish with an extra sprinkle of Parmigiano Reggiano.

Cover tightly with foil and bake for 25 minutes, until heated through.

Remove the foil and bake an additional 10–15 minutes, until the sauce is bubbling and the tops are lightly golden.

Let the shells rest 5–10 minutes before serving. Finish with fresh basil and another light snowfall of Parmigiano.