Place potatoes and 1 teaspoon salt in a Dutch oven; add water to cover. Bring to a boil. Reduce heat; cook, uncovered, for 10 minutes. Add celery and onion; cook 10-15 minutes longer or until vegetables are tender. Drain; transfer to a large bowl.
Mash potato mixture, gradually adding 3/4 cup cheese, milk, butter, pepper and remaining salt. Transfer to a greased 8-in. square baking pan; sprinkle with remaining cheese. Broil 3-4 in. from the heat for 5-8 minutes or until cheese is lightly browned. Yield: 12 servings (3/4 cup each).