Cook pasta according to package directions until just cooked al dente. Drain and let cool.
Meanwhile, heat spinach in the microwave until fully thawed. Let cool and squeeze out extra moisture by squeezing in a clean towel or placing in a fine mesh sieve and pushing with back of spoon.
In a large bowl, combine ricotta, egg, basil, oregano, garlic powder, salt, pepper, and spinach. Mix until well combined.
Spoon ricotta mixture into a gallon size zip top bag. Cut off a corner of the bag so the mixture can be squeezed through.
Add about half a jar of marinara to the bottom of each disposable casserole dish.
Take each shell and fill with ricotta mixture, and place in the casserole dish. Once all the pasta is filled, cover with remaining sauce, then sprinkle with mozzarella cheese.
Cover each dish then wrap tightly with plastic wrap and then aluminum foil. We want it to be well protected to avoid freezer burn or picking up flavors from surrounding items in freezer.
When ready to cook, you can let it thaw or place directly into the oven (after removing plastic wrap and foil, then covering with a single piece of foil). Add about an hour of time if cooking from frozen. If thawed, cook at 350 degrees until heated through and bubbly, about 45-50 minutes.
Alternatively, you could fill each shell with ricotta mixture then freeze until solid on a wax paper lined baking sheet. Place shells in freezer bag and squeeze out remaining air. When ready to cook, remove from freezer and build like we did above (marinara on bottom and top, then covered with cheese). You will need to increase the cooking time if cooking. from frozen.