Irish Cheddar and Potato Gratin

8 large potatoes, peeled and sliced 1/4" thick
salt and pepper
1 cup finely chopped spring onion (scallions)
2 cups shredded Irish Cheddar cheese
butter
1/2 cup chicken stock, hot

Preheat oven to 350°F. Coat an oven-proof gratin dish with butter. Layer alternately, the sliced potatoes, salt and pepper, onions, and cheese, finishing off with a layer of potatoes and reserving 1/2 cup of the cheese. Dot with butter. Pour hot stock over the dish. Butter a square of aluminum foil and cover the dish. Bake at 350°F for about 45 minutes. Remove foil, top with remaining cheese and bake an additional 15 minutes or until potatoes are done and cheese is melted and bubbly.