Preheat oven 375F.
Begin heating marinara sauce so that it's simmering when you are ready to assemble the manicotti. Then fill a large pot with salted water for the lasagna noodles and put it on high heat.
Puree ricotta cheese and spinach in a food processor until smooth, about 20 seconds. Put ricotta mixture into a large bowl. Add parmesan cheese, 1 1/2 cups of the mozzarella, eggs and salt. Mix well and set aside.
Once the water is boiling for the pasta, add the noodles and cook them for 5 minutes. Drain and toss with olive oil so they don't stick together.
Set out a 9x13 casserole dish and cover the bottom with marinara sauce, about 1 cup.
Lay out one of the noodles and put about 1/4 cup of the filling near the end of the noodle closest to you. Roll up the noodle from the end near you to the other end.
Put the rolled, filled noodle into the casserole dish. Pack the manicotti into the dish until you have them all in there.
Cover the manicotti thoroughly with marinara sauce, 2-3 cups. Then sprinkle the remaining mozzarella over the top.
Cover the casserole dish with foil and bake for 30 mins.
Remove the foil and bake for 10 minutes more. If desired, top with chopped basil or parsley.
Serve with more parmesan at the table.