Bring a large pot of salted water to a boil.

Add pasta shells and cook until al dente, about 10 minutes.

Drain well then toss with oil in a large bowl; set aside.

Preheat the oven to 375°F. Put spinach into a large skillet and drizzle with 1 to 2 tablespoons water.

Cover and cook over medium-low heat, tossing occasionally, until just wilted; drain well.

When cool enough to handle, squeeze to remove any excess water.

Chop spinach and transfer to a large bowl.

Add garlic, ricotta, mozzarella, egg and basil to make the filling.

Spread a few spoonfuls of marinara sauce in the bottom of a shallow baking dish large enough to hold the shells in a single layer.

Fill each shell with spinach mixture and arrange in the dish.

Pour remaining sauce over and around the stuffed shells and top with Parmigiano Reggiano.

Cover with foil and bake until hot throughout, 20 to 30 minutes, then uncover and continue baking until golden brown, about 10 minutes more.