In a large pot, add chicken and cover with water (about 8 cups). Boil for 30 minutes. Skim oil while cooking. Remove and throw out water.
In the same pot, heat oil and add chopped onions, white ends of scallions, ginger, and lemongrass. Sauté for about 2 minutes. Be careful not to burn them.
Pour in 8 cups of water. Bring to a boil. And chicken and cook, uncovered, over medium heat for about 30 minutes.
Add the potatoes and carrots. Cook for another 20 minutes.
Add spinach and remaining scallions. Cook for another 5 minutes.
Serve hot, ideally with white rice.