Heat the oil in a large saucepan over medium heat and cook the garlic and ginger until browned. Combine the broth, soy sauce, and fish sauce, if using, to the saucepan and bring to a simmer. Add the remaining ingredients except the snow peas, scallions, and sesame oil. Simmer for 20 minutes, and then add the snow peas. Cook for just a few minutes longer, until the snow peas have softened but are still bright green. Add the scallions to the saucepan, remove from heat, and ladle into serving bowls. Drizzle a bit of sesame oil over the top of each serving.