Peel the onion and carrot. Rinse the carrot and celery under cold water.
Put enough water in a pan to cover the fish later. Add onion, carrot, and celery and bring to a boil.
Add the fish, vinegar, and salt. Cover the pan. When the water returns to a boil, adjust heat to a gentle, steady simmer and cook about 8 minutes, depending on fish thickness.
Lift the fish out with a slotted spoon or spatula and set it on a platter. Remove skin, any gelatinous matter, the center bone, and all small bones you can find.
Mash the fish with a fork. Mix in egg yolks, grated Parmigiano-Reggiano, marjoram, a tiny grating of nutmeg (about 1/8 tsp), a few grindings of pepper, and the cream. Mix until evenly combined. Taste and adjust salt.
Make yellow pasta dough by the machine method or hand-rolled method. Cut, stuff, and shape tortellini. When boiling, add 1 tbsp olive oil to the water.
