Preheat the oven to 350 degrees Fahrenheit.
In a large pot, bring water to a boil (salt with about 1 tablespoon) on high heat. Once the water is boiling, add the shells, and stir often so they don’t stick together. Cook for 8-9 minutes until al dente.
While the pasta boils. Squeeze the thawed spinach removing all the liquids out of it.
In a large mixing bowl combine ricotta cheese, drained spinach, crushed garlic, Parmesan reggiano, ¾ cup mozzarella, egg, Italian seasoning, salt, and pepper. Mix until it’s well blended.
Drain shells, and rinse well under cold water bringing the shells to room temperature. Begin to separate the shells onto a paper towel lined surface.
Spray a 9x11 baking dish with olive oil spray. Add one cup of sauce to the pan and spread around so the bottom is just covered.
Take a single shell pasta and scoop the filling using a teaspoon into the shell. Once filling is in shell, gently push the seems shut as best as possible. Place the shell seam side up and repeat this step until all shells are filled.
Pour the remaining marinara sauce over the shells making sure all of the shells are covered. Sprinkle the remaining ¼ cup mozzarella cheese on top.
Cover with aluminum foil and bake for 30 minutes. Remove aluminum foil after time is up and bake for an additional 5-10 minutes or until cheese is golden brown.