Preheat the oven to 300 F. Butter a soufflÄ dish. Melt 1 tablespoon of the butter in a frying pan and cook the mushrooms until softened, about 5 minutes. Drain. Mix together the mushrooms and ham. In a medium saucepan scald the milk. Season the milk with nutmeg and salt and pepper. In a large saucepan melt the remaining butter and whisk in the flour and potato or cornstarch. Cook fo 2 minutes, whisking constantly. Pour in the milk and bring to a boil, stirring constantly. When the sauce is thickened, remove the pan from the heat and whisk in in the egg yolks one at a time. Beat in the ham and mushrooms. Beat the egg whites until stiff and fold them into the sauce. Spoon the mixture into the prepared dish and bake for 15 minutes. If the the top is too dark, cover it with a paper towel. Bake for 15-20 minutes longer.
