Put a large sauté pan over a medium flame and heat the oil. Add the onion and sauté until soft, about 3-5 minutes.
Add the passata and salt to the pan and cook for 5 minutes.
Add the water and the peas to the pan. Return to a boil and taste for seasoning. Season with more salt as desired.
Drop the elbow macaroni into a pot of generously salted boiling water and cook for 3 minutes less than the package instructions, about 4-5 minutes. The pasta should still be a little bit hard.
Drain the pasta and transfer to the pot with the tomato. Cook for 1-2 minutes.
Serve immediately with plenty of grated cheese.