Cook potatoes in water until almost tender; drain. In a bowl, beat eggs; gradually add cheese spread and milk (mixture will appear lumpy). Stir in potatoes, ham and half the onions. In a 10-in. ovenproof skillet, melt butter over low heat; remove from the heat. pour egg mixture into skillet. Bake, uncovered, at 350° for 20 minutes. Sprinkle with remaining onions; bake 5-10 minutes longer or until a knife inserted in the center comes out clean. Let stand for 5 minutes before cutting.