Combine the breadcrumbs and heavy cream in a small bowl, stirring with a fork until all the crumbs are moistened. Set aside.
Heat the oil in a small skillet over medium heat. Add the onion and sauté for about five minutes, until softened. Remove from heat.
In a large bowl, combine the ground beef, ground pork, onion, sugar and egg, salt, and pepper and mix well with your hands.
Add the breadcrumb-cream mixture and mix well. With wet hands (to keep the mixture from sticking), shape the mixture into meatballs the size of a golf ball, placing them on a plate lightly moistened with water. You should have about 24 meatballs.
Melt the butter in a large skillet over medium-high heat. Add the meatballs, in batches if necessary, and cook, turning frequently, for about 7 minutes until browned on all sides and cooked through. Transfer the meatballs to a plate and drain off all but 1 tablespoon of fat from the skillet.