In a small saucepan, cook the potato in salted boiling water until tender, about 12 minutes. Drain well, then pass through a ricer into a bowl. Let cool.

Meanwhile, preheat the oven to 425°. In a large bowl, whisk the egg with the milk and cream. Stir in the breadcrumbs and let stand for 5 minutes to soften. Add the beef, pork, onion, salt, pepper and riced potato and mix just until combined. Shape the mixture into 18 meatballs.

In a large ovenproof nonstick skillet, melt 2 tablespoons of the butter. Add half of the meatballs and cook over moderate heat, turning, until golden all over, about 8 minutes. Transfer to a plate and wipe out the skillet. Repeat with the remaining 2 tablespoons of butter and meatballs.

Return all of the meatballs to the skillet and bake until cooked through, about 10 minutes. Transfer to a platter and serve with the Quick-Pickled Sweet Cucumbers, mashed potatoes and lingonberry jam.