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 30 to 45 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 but not cooked through.
Add tomatoes and cook, mashing regularly with back spoon until softened and release juices.
Add fish sauce and cook for about 1 to 2 minutes.
Add water and bring to a boil.
Lower heat, cover, and cook for about 20 to 30 minutes or until chicken is cooked through and sauce is thickened to desired consistency.
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.
Season with salt and pepper to taste. Serve hot.