Saute onion for 3-5 minutes in olive oil on medium-low heat in a large pot until soft. Add garlic and saute for 2-3 minutes more until fragrant.
Add the tomato paste and cook for 5 minutes, stirring with a spoon. Add the plum tomatoes and season with pepper.
Cook for 15 minutes uncovered, stirring frequently to avoid any sticking. After 15 minutes, taste-test the sauce and adjust salt and pepper if required.
Keep the sauce warm and set aside.
Preheat oven to 375f. Boil pasta to 1-2 minutes less than package instructions.
In a large bowl mix pasta with 2-3 cups of sauce and 1 pound of ricotta cheese. Mix until smooth and creamy.
Coat a 4-5 quart baking dish with a thin layer of sauce. Add ⅓ of the pasta. Sprinkle ⅓ of the cubed mozzarella, ⅓ of the grated Pecorino Romano and a bit more sauce on top. Repeat for another 2 layers.
On the top layer add a bit more sauce, the shredded mozzarella, and the remaining Pecorino Romano cheese.
Bake for 25-30 minutes or until the cheese is bubbly and lightly brown. If a more crispy top is desired, broil for 1-2 minutes, but watch carefully to avoid burning.
Remove from oven and let baked ziti sit for 5-10 minutes to settle. Serve with remaining sauce on the side, more grated cheese, and Italian bread. Enjoy!
