Whisk the egg yolk in a bowl, mix well with the ground beef and add a pinch of salt and freshly ground black pepper.
Shape small balls, ideally 1cm diameter.

Finely chop the onion, garlic, carrot and celery. Gently fry in two tablespoons of olive oil for 5 minutes.
Add the meatballs and cook until golden. Remove the meatballs, add the passata and cook for about 20 minutes.
Put the meatballs back in the tomato sauce.

Cook the pasta al dente in salted water, drain and put in the tomato sauce with the meatballs.
Quickly sautè in the pan, and serve with grated Parmigiano Reggiano.

The “chitarra” (guitar) is a stringed tool (hence it’s name) that we use in Abruzzo to cut hand-made spaghetti with a unique square profile.