In a 6 quart stockpot on medium heat, cook onions in olive oil until tender and translucent.
Add garlic and saute until fragrant (approximately 30 seconds)
Add basil, oregano, italian seasoning, tomato sauce, and diced tomatoes. Season with salt and pepper.
Simmer on medium low for 30 minutes.
While marinara is simmering, cook manicotti shells in boiling water for 7 minutes.
In a large mixing bowl, combine ricotta, parmesan, mozzarella, parsley, oregano and eggs.
Once shells are cooked and cheese is mixed, pour half of the marinara into a 9x13 baking dish.
Pour cheese mixture into a gallon size storage bag. Cut a small corner off the cheese filled storage bag. This is a quick makeshift, disposable piping bag.
Preheat oven to 350 degrees.
Fill manicotti shells with the cheese mixture by squeezing it into the shells from the corner of the bag. Fill half the shell, then turn it around and fill the other half. Continue to fill each shell and place them on top of the marinara sauce.
Once all shells are filled and in the pan, cover them with the remainder marinara sauce. Sprinkle the mozzarella cheese and parmesan cheese over the top and cover with foil.
Bake the manicotti, covered, for 40 minutes. Remove the foil and continue to bake for 10-15 minutes longer, or until the top is browning and the marinara is bubbling.
Remove from oven and allow to rest for 10 minutes before serving.