Step 1

Bring chicken, rice, garlic, ginger, and 5 cups water to a boil in a large saucepan. Add a big pinch of salt. Reduce heat to medium-low and simmer, uncovered and stirring occasionally, until rice has swelled and chicken is firm, 10–12 minutes.

Step 2

Add potatoes to pan and cook over medium-low heat, still uncovered and stirring occasionally, until soup is thickened and potatoes and rice are tender, 15–20 minutes longer. Transfer chicken to a bowl and shred with 2 forks, then return to pot. Stir in lemon juice and soy sauce; taste soup and season with salt if needed.

Step 3

Divide soup among bowls. Top with cilantro and lots of black pepper.

Step 4

Do Ahead: Soup (without cilantro) can be made 3 days ahead. Transfer to an airtight container and chill. Reheat over medium-low, adding water to thin as needed.