Swedish Meatball Soup

Name: Swedish Meatball Soup
Ingredients: 2 cups finely chopped yellow onion (about 1 large ), divided; 1 tablespoon water; 2 large eggs , lightly beaten; 1/2 cup whole-wheat panko breadcrumbs; 1/3 cup whole milk; 4 teaspoons reduced-sodium Worcestershire sauce , divided; 1 teaspoon ground pepper , divided; 3/4 teaspoon ground allspice , divided; 1/4 teaspoon ground nutmeg plus 1/8 teaspoon , divided; 1 pound lean ground beef; 3 tablespoons extra-virgin olive oil , divided; 2 tablespoons all-purpose flour; 4 cups reduced-sodium beef broth; 1 large russet potato , scrubbed and cubed (1/2-inch); 12 ounces sliced cremini mushrooms; 2 teaspoons cider vinegar; 2 teaspoons granulated sugar; 1 1/2 teaspoons Dijon mustard; 3/4 teaspoon salt; 1 cup heavy cream; 2 tablespoons finely chopped fresh flat-leaf parsley