1. Over medium heat, melt the butter in a small skillet. Once the foam has subsided, cook the onions for about 5 minutes; they should be tender and translucent, but not browned.
2. In a large bowl, combine the onions, mashed potato, breadcrumbs, meat, cream, salt, and egg (and parsley if desired).
3. Mix everything together vigorously (using your hands is perfect) until the mixture is homogeneous. Shape into small meatballs and place them on a baking sheet, then cover with plastic wrap. Let the meatballs rest in the refrigerator for one hour before cooking.
4. Over high heat, melt the butter and oil in a large skillet. Once the foam has subsided, add the meatballs, 8 or 10 at a time. Reduce the heat to medium and fry the meatballs while shaking the pan to ensure that the meat rolls well in the fat and that the meatballs retain their shape.
5. After 8 to 10 minutes, the meatballs should be browned on the outside and no longer pink in the center.
6. Add more butter and oil as needed, and transfer each new batch of meatballs to a dish that you will keep in the oven at 200°C.
7. If desired, the cooking juices can be used to make a sauce (with flour).