Prepare and cook the asparagus in salted water until tender.
Drain well.
Arrange the asparagus in the same direction in a shallow heatproof dish.
Spread about 1 cup of Sauce Mornay over the asparagus.
Sprinkle lightly with grated Parmesan or Gruyère cheese.
Drizzle with a small amount of melted butter.
Place under a hot broiler until the top is lightly browned and bubbling.
Serve immediately.