Peel potatoes and cut into thin strips. Place in a bowl of ice water.
For cajun sauce: Saute onion, peppers, garlic and celery in vegetable oil (preferably peanut oil). When vegetables are tender, whisk in flour, and 2 tsp cajun seasoning. Gradually whisk in beef stock and cook until thick and smooth. Cool and reserve sauce for hot fries.
Drain potatoes. Heat vegetable oil in deep fryer or in a 4qt. saucepan to 375 degrees. Place fries in a fryer basket and fry until golden brown, about 5-10 min. Drain on paper towel sprinkle with 2 tbsp. cajun seasoning, and serve with heated cajun sauce.