Preheat oven to 350 degrees.
Cook pasta shells in boiling water for 6-7 minutes. Remove, drain and set aside.
Heat olive oil in saute pan and cook garlic and 4 cups of packed spinach (remove large stems) for a few minutes until the spinach starts to wilt.
Add the spinach and garlic mixture to a large bowl containing the ricotta cheese, 1 cup of the mozzarella, salt, pepper, and Italian seasoning. Mix until well combined.
Stuff the shells with the cheese and spinach mixture.
Spread ½ cup marinara sauce on the bottom of a 9x13 casserole dish. Add stuffed shells so you have four rows of five shells.
Top shells with remaining 1 cup of mozzarella and remaining 1 cup of marinara sauce.
Bake at 350 degrees for 30 minutes.