Prepare a broth by combining water, ⅔ of the tomato passata/purée (1:1 ratio), tomato paste, and salt. Bring the mixture to a boil, then let it simmer. The broth should be bright red and flavorful but still have a thin, broth-like consistency. Make sure the broth is hot when you add it to the spaghetti; otherwise, it will lower the cooking temperature and affect how the pasta cooks.
In a saucepan, heat the olive oil and garlic cloves. Adjust the amount of red chili pepper based on how spicy you want your spaghetti. Cook the garlic over high heat until it turns golden. Next, pour in the undiluted tomato purée.
Spread the passata evenly over the pan using a wooden spoon, and let it reduce and thicken slightly. Then, place the uncooked spaghetti on top of the sauce, arranging it so that it rests on the surface.
Wait until the underside of the spaghetti begins to caramelize before turning it over. Do this carefully, a little at a time, using a spatula. Allow the other side to crisp up slightly, then pour a ladle of hot tomato broth around the edges of the pan, not directly over the spaghetti. As the spaghetti softens, gently stir it with a wooden spoon to move any strands stuck to the bottom of the pan and help the broth spread. When the broth has reduced and the spaghetti starts to stick again, add more broth as needed.
Continue adding tomato broth and stirring the spaghetti until it’s well-coated with the sauce and cooked through. If you run out of tomato broth, you can add a bit of water.
Serve your Spaghetti all’Assassina immediately, topped with Parmigiano, stracciatella, or simply on its own.