Preheat the oven to 375°F.
Heat 2 tablespoon olive oil in a skillet.
Add the ground beef. With a wooden spoon, break apart any clumps. Season with ½ teaspoon kosher salt and ½ teaspoon freshly cracked black pepper.
After browning the ground beef for about 5 minutes over medium heat, add the chopped onions and mix. Cook for another 5 minutes, until the ground beef has nicely browned and the onions become translucent.
Add the minced garlic for just a minute.
Add the whole can of plum tomatoes (including any juices) and 3 ounces of tomato paste. Bring to a simmer. Stir regularly and use tongs to help break apart the plum tomatoes.
In between stirring the sauce, finely grate 4 ounces of Pecorino Romano and cut 8 ounces of mozzarella into ½“ cubes.
Keep simmering and stirring the pot — smell, taste. Be careful with the salt. If it tastes just slightly under-salted, that’s perfect. The salty Pecorino Romano will close that gap when added before baking.
In the meantime, bring a pot of salted water to boil, add 8 oz rigatoni, and cook. Before draining the pasta, scoop out ¼ cup of the pasta water and add to the sauce.
Now we’re ready to put everything together and finish cooking in the oven. If you made the sauce in an oven-safe skillet, add the grated Pecorino Romano cheese and the cooked pasta. Mix well. Top with cubed mozzarella and in the oven it goes. Bake for 30 minutes. Otherwise, transfer the sauce to a baking dish, mix everything as above, and bake.
Once it’s ready to serve, remove from the oven and start heaping it on plates. Garnish with some roughly chopped fresh Italian flat-leaf parsley.