Melt the butter in a skillet, stir in the flour and add the milk gradually, stirring constantly until the sauce is smooth and thick. Add the cheese and stir until melted. Add the celery, pimiento, mushrooms, ham and well beaten egg yolks, curry powder, and onions. Season to taste. Cool to lukewarm arid fold in the egg whites, beaten until stiff. Turn into a well greased casserole and bake in a 350 degrees F oven for 35-40 minutes until firm and well browned.