2 large onions, halved lengthwise and thinly sliced crosswise
2 tablespoons light brown sugar
Two 14 1/2-ounce cans diced tomatoes in their juices
4 cups beef stock or low-sodium broth
2 tablespoons fresh lemon juice
Kosher salt and freshly ground pepper
1 pound ground pork
1 large egg white
1 1/2 teaspoons cornstarch
Finely grated zest of 1 lemon
2 tablespoons coarsely chopped cilantro
2 scallions, white and green parts, finely chopped
In a medium soup pot, heat the olive oil until shimmering. Add the onions, cover and cook them over moderately high heat, stirring occasionally, until they are softened, 3 to 4 minutes. Uncover and cook until the onions are lightly browned, about 5 minutes longer. Add the brown sugar and cook for 1 minute. Add the tomatoes and their juices, the beef stock and lemon juice, season lightly with salt and pepper and bring to a boil. Simmer the soup over moderate heat for 10 minutes.
Meanwhile, in a medium bowl, combine the ground pork with the egg white, cornstarch, lemon zest, 2 teaspoons of kosher salt, 1 teaspoon of ground pepper and half of the chopped cilantro and scallions. Using lightly moistened hands, roll slightly rounded tablespoons of the meat mixture into balls.
Add the meatballs to the soup, cover and cook them over moderate heat until they are cooked through, about 5 minutes. Stir in the remaining cilantro and scallions. Ladle the soup into deep bowls and serve right away.