Swedish Meatballs with Mashed Potatoes (Köttbullar med Potatismos)

Ingredients

• 4 lb. (about 5 large) russet potatoes, peeled and cut into 1″ chunks
• 2 ¹⁄₄ cups heavy cream
• 14 tbsp. unsalted butter, cubed
• Kosher salt and freshly ground black or white pepper, to taste
• 2 slices (about 3 oz.) crustless white bread, torn into small pieces
• 12 oz. ground pork
• 12 oz. ground beef
• ¹⁄₄ tsp. ground allspice
• 1 egg, lightly beaten
• ¹⁄₂ small yellow onion, minced
• 3 tbsp. flour
• 3 cups beef stock
• Lingonberry preserves, for serving