Prepare the tomato sauce as per recipe. In a saucepan bring the broth to a simmer. Place the tomato sauce in a larger saucepan on medium heat and bring to a boil.
Stir the rice into the tomato sauce and turn the heat to medium.
When the rice starts absorbing the sauce and appears thicker, add a ladle of broth.
Stir frequently to prevent the rice from sticking to the pan.
When the broth is absorbed and the rice becomes thicker, add more broth. Continue stirring and adding broth, a ladle at a time, keeping the rice to the consistency of a dense paste. Add salt and pepper. The rice is ready in about 22 minutes, when “al dente”: firm, but not too soft and overcooked.
Beat the egg with a pinch of salt. When the rice is ready add the egg and mix vigorously.
Stir in the parmigiano cheese and butter. Transfer to warm serving dishes and serve at once.