Add cubed chicken breast to a large Dutch oven or soup pot on medium-low heat with grated ginger, diced onion, and a tablespoon of cooking oil. Stir for 1-2 minutes until the outside of chicken begins to lightly brown. Add garlic.
Add coconut milk, chicken broth, potatoes, and lemongrass stalks (crush them lightly with a mallet to release the flavor; you'll remove them when soup is finished).
Add remaining ingredients, cover, and continue cooking for 30-40 minutes.
Garnish with sliced green onion and/or basil and enjoy!