In a large pot, sauté the chopped onion in olive oil until soft.
Add the frozen peas, salt, pepper, and water. Simmer for 5 minutes.
Remove half of the peas and blend until smooth. Pour it back in! This is the secret to how to cook peas so they become a rich sauce.
Add your Ditalini and cook for 10 minutes, stirring frequently. Add a splash of water if it gets too dry.
Turn off the heat. Stir in the Pecorino Romano until it creates a thick, cheesy sauce.