8 large potatoes, peeled and cubed (about 4 pounds)
1-1/2 teaspoons salt, divided
2 cups chopped celery
3/4 cup chopped onion
1-1/2 cups shredded Swiss cheese, divided
2/3 cup 2% milk
3 tablespoons butter
1/4 teaspoon pepper
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 until vegetables are tender, 10-15 minutes. 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 until cheese is lightly browned, 5-8 minutes.