In a pan over medium heat, heat oil.
Add potatoes and carrots and cook, turning once or twice, until lightly browned. Remove from pan and drain on paper towels.
Remove excess oil from pan except for about 2 tablespoons.
Add bell peppers and cook for about 10 to 15 seconds. Remove from pan and drain on paper towels.
Add onions and garlic and cook until softened.
Add chicken and cook, turning on sides once or twice, until lightly browned.
Add tomatoes and cook, mashing regularly with back spoon until softened and release juices.
Add fish sauce and simmer for about 30 to 40 seconds.
Add water and bring to a boil.
Lower heat, cover, and cook for about 15 to 25 minutes or until chicken is tender and sauce is slightly reduced.
Add potatoes and carrots and continue to cook until fork-tender.
Add bell peppers and sweet peas. Cook for 1 to 2 minutes or until green peas are heated through and bell peppers are tender yet crisp.
Continue to simmer until sauce is reduced to desired consistency. Season with salt and pepper to taste. Serve hot.