Heat the oven to 375°F. Boil the ziti in salted water until 2 minutes shy of the package's al dente time, then drain.

Warm the olive oil in a skillet over medium heat. Cook the onion for 4 to 5 minutes until soft, add the garlic for 30 seconds, then stir in the tomato paste for 1 minute. Add the crushed tomatoes, dried basil, 1/4 teaspoon salt, and black pepper. Simmer for 8 minutes.

Stir the ricotta, egg, oregano, red pepper flakes if using, parsley, and remaining 1/4 teaspoon salt in a bowl.

Toss the drained ziti with about 1 cup of sauce. Spread half in a 9 x 13-inch baking dish, add half the ricotta mixture in spoonfuls, and sprinkle with half the mozzarella and Parmesan. Repeat, then spread the remaining sauce over the top.

Cover the dish with foil and bake for 20 minutes. Uncover and bake 10 minutes more, until the sauce bubbles around the edges and the center reaches 165°F.

Let the baked ziti rest for 10 minutes so the layers settle, then serve warm.