Preheat your oven to 375F and move the rack to the top third of the oven.
Boil a large, salted pot of water and cook the lasagna noodles al dente according to package directions. Once they're done, drain them (don't rinse them), and as soon as they're cool enough to handle, lay them flat (beside each other, not overlapping) on a piece of parchment paper or foil.
Meanwhile, add about 1 cup of the marinara sauce to the bottom of a deep 9x13 casserole dish and spread it in an even layer.
Thaw the spinach and squeeze the water out thoroughly. Add it to a prep bowl along with the parmesan, ricotta, 1 cup of the mozzarella, egg, salt & pepper, garlic, Italian seasoning, and oregano. Stir together.
If the lasagna noodles are a bit wet, pat them dry with paper towel. Spread about 1/3 cup of the cheese/spinach mixture onto each noodle, and then roll it up (I roll it away from myself), add it to the casserole dish (seam side down), and repeat until all 12 noodles are done.
Spoon the remaining marinara sauce over top and sprinkle on the rest of the mozzarella. Cover with foil and bake for 30-35 minutes or until hot and bubbly. Optional: take the foil off and brown the cheese for a few minutes (watch it carefully).
Let it rest for a few minutes and then serve with freshly chopped parsley and/or basil sprinkled over top and extra salt & pepper if needed.