In a bowl, put the breadcrumbs with the cream and mix.
Heat the butter to sauté the onion until it is nicely golden.
Put the ground beef and lamb in the bowl with the breadcrumbs, salt, pepper. Add the onion and the egg and mix well.
With your hands, form meatballs the size of a golf ball and refrigerate for 15 minutes.
Melt the rest of the butter in a pan. Cook the meatballs in batches. You have to cook them on all sides. Set aside the cooked meatballs on a plate.
Remove the excess fat and the small pieces of meat that may have burned before preparing the sauce.
Pour in the beef broth and the cream. Add the cornstarch previously diluted in a little water.
Add a pinch of nutmeg to the sauce. Salt, pepper.
Put the meatballs back into the sauce and serve with mashed potatoes.