Place potatoes and 1 tsp salt in a large pot; 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" square baking pan; sprinkle with remaining cheese.

Broil 3-4" from the heat for 5-8 minutes or until cheese is lightly browned.