Cook the pasta shells according to the directions found on the package and then drain, run under cold water, and drain again.

While the shells are cooking, stir together the ricotta cheese and feta. In a skillet, heat the butter or oil and very lightly saute the spinach just until it wilts, 1-2 minutes over medium heat. Stir the spinach into the cheese. Add the eggs and stir well. Add the nutmeg, and season with salt and pepper and stir well.

Preheat the oven to 350 degrees. Prepare a 9x13 pan by spreading just enough tomato sauce to cover the bottom of the pan.

Begin stuffing the shells by holding them open in one hand and adding a spoonful of filling. Place shells close together in the prepared pan, repeating with the rest until all the shells are stuffed. Pour the remaining marinara sauce over top.

Place the stuffed shells in the oven and bake for 30-35 minutes until the sauce is bubbly. Remove from the oven and serve immediately with Parmesan cheese for topping.