Heat oil in a Dutch oven over medium heat. Add potato, 1 cup onions, pepper, celery, and peas to pan; cook 8 minutes, stirring frequently. Add lemongrass, ginger, and garlic; cook 1 minute, stirring frequently. Stir in broth; bring to a boil. Cover, reduce heat, and simmer 10 minutes. Add chicken, juice, chili garlic sauce, and milk; cook 1 minute or until thoroughly heated. Sprinkle with remaining 1/4 cup onions and cilantro; serve with lime wedges.