Line a rimmed baking sheet with kitchen parchment and mist with cooking spray. In a medium bowl, combine the pork, scallion whites, egg white, 1 tablespoon of the fish sauce, 2 teaspoons of the ginger, 1¼ teaspoons kosher salt and 1 teaspoon white pepper.

Mix with your hands. Lightly moisten your hands with water and form into 20 balls, each about a generous tablespoon, and set on the prepared baking sheet. Cover and refrigerate.

In a large Dutch oven over medium, heat the oil until shimmering. Add the onion and cook, stirring occasionally, until beginning to soften, about 5 minutes. Add the remaining 2 teaspoons ginger and the garlic, then cook until fragrant, about 30 seconds.

Add the broth and bring to a boil over high. Reduce to medium-low and simmer, uncovered, until the onion is fully softened, about 10 minutes.

Add the meatballs, bring to a simmer over medium-high, then reduce heat to maintain a simmer. Cook undisturbed until the meatballs are cooked through, 160°F at the center, 8 to 10 minutes; cut through a meatball to check for doneness.

Off heat, stir in the watercress and remaining 2 tablespoons fish sauce. Let stand until the greens are wilted and tender, about 1 minute. Stir in the lime juice. Taste and season with salt and pepper, then stir in the scallion greens.