Saute the onion in the oil in a medium size saucepan until tender
Add the tomato paste tomato sauce water oregano basil salt and pepper Simmer covered for 10 minutes Preheat the oven to 375 Coat the inside of a 1 12-quart casserole with nonstick cooking spray Combine the Ricotta and half of the Mozzarella in a large bowl Add the cooked ziti and half of the tomato sauce
Stir gently to mix Turn into the prepared casserole
Add the remaining tomato sauce Sprinkle with the remaining Mozzarella cheese
Bake in the preheated oven for 25 to 30 minutes or until the cheese is melted and browned