Heat the olive oil in a pot and add diced onions. Saute onion until soft about 3-4 minutes.
Add the ground beef and cook until the meat is brown and cooked through. Drain off fat but leave some for moisture.
Add spaghetti sauce. Stir and simmer for 10-15 minutes. Turn off heat and let it cool.
Meanwhile, bring a large pot of water to a boil. Add salt and ziti. Cook pasta until al dente about 8 minutes. Do not overcook.
Drain ziti in a strainer and rinse with cold water to stop cooking.
You may add a tbsp of olive oil, so pasta won't stick together.
Preheat oven to 375 degrees F.
Pour half of the meat sauce in the bottom of 9x13 inch baking dish.
Add 1/2 of the ziti over the meat sauce then add ricotta cheese. Spread it out evenly. Sprinkle 1 cup of mozzarella cheese on top. (See notes for a different process)
Then add another layer with the rest of the ziti and and meat sauce. Add the remaining mozzarella and parmesan cheese on top.
Bake until bubbling, about 30-35 minutes. Let stand for 5 minutes before serving.