Cut the chicken breast into small pieces. Put a few dashes of fish sauce (around 1-2 teaspoons, enough to give a little taste but not overwhelming). Marinate while you cut the mango.
Peel the mango and cut into bite-size pieces. Cut the tomatoes into eighths.
In a wok on medium heat, cook the chicken until it's done. Then add the mangoes, cook until they are charred and have released some of their juices (but are not mushy).
Add the tomatoes. Cover for 1 minute or so. Uncover: the tomatoes should be a bit wrinkled.
Serve with rice.
Recipe by Aliette de Bodard at https://www.aliettedebodard.com/recipes/ga-xao-xoai-ca-chua-mango-tomato-chicken/