Cook pasta al dente according to directions on the box. Drain and set aside.
In a large mixing bowl combine cheeses, egg, chopped basil, salt and pepper. Set aside.
In a small skillet over medium heat place a tbsp of oil in a skillet and sautee spinach for 2 minutes. Season with salt and pepper. After spinach is wilted, grate in garlic and sautee and stir for another minute. Remove from the heat and let cool slightly.
Stir spinach into ricotta mix.
In the bottom of a large skillet or casserole skillet spread a cup of marinara sauce.
With a spoon stuff the shells with the ricotta mixture and place in the prepared skillet making a circle around the edges and working your way in. You may have a few extra shells left over.
Once all the shells are stuffed and placed in the pan pour the remaining sauce on top and sprinkle with a 1/2 cup of mozzarella cheese.
Place lid on the skillet and cook over medium-low heat for about 25 minutes.