Heat oil in a large pot over medium heat. Cook and stir onion, garlic, and ginger in hot oil until fragrant, about 5 minutes. Add chicken wings; cook and stir for 1 minute. Stir in fish sauce, cover, and cook another 2 minutes.
Pour chicken broth into the pot. Stir in sweet rice and bring mixture to a boil. Cover and cook for 10 minutes, stirring occasionally to prevent rice from sticking to the bottom of the pot. Season with salt and pepper.
Portion soup into bowls and garnish with green onion. Serve with lemon slices and additional fish sauce, if desired.
