Mix the diced lardo and onion on a cutting board, then mince them together into a fine paste with a knife or, even better, a mezzaluna.
Place the resulting mince in a saucier, sauté pan or flat bottomed wok, along with a drizzle of olive oil or a dollop of lard if using. Sauté the mince very gently until the lardo has rendered its fat and the onion is soft and translucent. Take care not to brown the ingredients.
Add the cherry tomatoes, passata, a good pinch of salt and, if you like, a pinch of red pepper flakes. Simmer for 15-20 minutes, until the tomatoes have melted into a velvety, glossy sauce but still a little chunky. Add the fresh basil leaves towards the end of cooking.
While the sauce is simmering, bring a large pot of well salted water to a boil. If using traditional long ziti (see Notes below), break them into lengths. Boil the ziti until just slightly underdone.
Transfer the ziti to the pan with the sauce, along with a ladleful of the pasta water. Simmer for another minute or two, until the pasta is al dente. Add more pasta water if things get too thick or dry.
Serve right away, topped with the grated cheese.