Preheat the oven to 350 degrees Fahrenheit.
Cook the pasta according to package directions, until tender when poked with a fork. Add a few sprinkles of salt to the water after it boils and just before adding the pasta. Drain the pasta when done and set aside.
Add olive oil, diced onions, and minced garlic to a skillet and stir and cook over the stove top until the onions are tender and translucent.
Add the cooked onions to a large bowl. Add the ricotta, chopped spinach, Italian seasoning, rosemary, salt, black pepper, shredded parmesan, and 1 c. shredded mozzarella. Stir until completely combined.
Spray a 9"x13" pan with nonstick cooking spray.
Fill each jumbo shell with the cheese mixture. Make sure it's not overflowing or overfilled inside the shell.
Squeeze as many filled shells together as you can in the prepared pan, then add a few on top, if necessary.
Evenly distribute the frozen meatballs on top of the filled shells.
Pour the marinara sauce evenly over the top.
Cover the pan with an oven-safe lid or foil (tent the foil a little, if necessary, so the sauce doesn't transfer to the foil).
Bake for approximately 40-45 minutes, or until completely heated through, bubbly, and the meatballs are nice and warm.
Sprinkle the remaining 2 c. shredded mozzarella evenly over the top and broil for just a few minutes, or just until the cheese starts turning slightly brown in places. Watch it carefully so it doesn't burn.
Enjoy!