In a pan over medium heat, heat about ¼ cup of oil. Add potatoes and carrots and cook until lightly browned. Remove from pan and drain well.
Remove excess oil except for about 1 tablespoon. Add bell peppers and cook for about 30 to 40 seconds. Remove from pan and drain on paper towels.
Add onions and garlic and cook until softened.
Add chicken and cook until lightly browned and juices run clear.
Add fish sauce and cook for about 1 minute.
Add tomato sauce, water, and chicken cubes. Stir to combine and bring to a boil.
Lower heat, cover, and simmer until chicken is tender and sauce is slightly reduced.
Add potatoes and carrots and cook until tender.
Add bell peppers and brown sugar. Season with salt and pepper to taste and stir well.
Add all-purpose cream and simmer until heated through. Serve hot.