Bring a large pot of salted water to a boil. Add potatoes and cook until tender but still firm, about 15 minutes. Drain and cool.
Combine potatoes, ham, corn and parsley; set aside. In a saucepan saute onion in butter for 2 minutes, stir in flour until blended well. Gradually add milk and pepper. Bring to a boil. Cook and stir for 2 minutes. Remove from heat and pour over the ham mixture. Stir to mix well.
Pour into greased 11x7 baking dish. Cover and bake for 25 minutes. Uncover, sprinkle with cheese and bake 5 to 10 minutes longer until cheese melts.
