Place the potatoes, celery and onion in a large saucepan; cover with water. Bring to a boil; cover and cook just until tender, about 15 minutes.
Meanwhile, melt butter in a small saucepan; stir in the flour, salt, celery seed and pepper until smooth. Gradually stir in milk. Bring to a boil; cook and stir for 2 minutes or until thickened.
Transfer sauce to a large bowl; stir in the sour cream, 1/2 cup cheddar cheese, process cheese and 3 tablespoons chives. Drain potato mixture; add to sauce and gently stir until coated.
Transfer to a 3-qt. baking dish coated with cooking spray. Cover and bake at 350° for 35-40 minutes or until bubbly. Sprinkle with remaining cheese; cover and let stand for 5 minutes or until cheese is melted. Sprinkle with remaining chives. Yield: 10 servings.