The Best Homemade Swedish Meatballs

Name: The Best Homemade Swedish Meatballs
Ingredients: 3 slices white bread , crusts removed, cut into cubes (about 2 cups); 1/4 cup milk; 4 tablespoons butter, divided; 1/2 large yellow or white onion , grated using the large holes of a box grater; 1 large egg; 3/4 pound ground beef; 1/2 pound ground pork; 1 teaspoon kosher salt; 1 teaspoon black pepper; 1/2 teaspoon freshly grated nutmeg; 1/2 teaspoon ground cardamom; 3 tablespoons butter; 3 tablespoons flour; 2 cups beef stock; 1/4 cup sour cream (use full-fat or the sauce may curdle); 2 tablespoons lingonberry, red currant, raspberry, or cranberry jelly , to taste (optional); Salt and pepper, to taste