Heat 1 tablespoon of oil over medium high heat. Place the chicken pieces and cook without disturbing until it is brown, about 3 minutes. Turn over to cook the other side.
Add the onion and stir frequently until tender, about 3 minutes. Mix in the garlic and continue to stir until fragrant, about 1 minute.
Stir in fish sauce, diced tomatoes, tomato paste, water, sugar and salt.
Add potatoes and carrots then bring to a boil. Cover the pot then reduce heat and simmer for 15-20 minutes, or until vegetables are tender, stirring occasionally and adding a little water if needed.
Adjust seasoning with salt and pepper and serve immediately.