1. Melt butter in a small saucepan. Whisk in flour. Gradually whisk in milk. Stir continuously, and bring to a boil. Reduce heat and simmer gently for 1 min. Whisk in Parmesan, mustard powder, hot sauce and salt. Remove from heat; the sauce will thicken slightly while preparing the remaining ingredients.
2. Toast bread. Poach eggs in simmering water for about 3 min.
3. Place one slice of ham on each slice of bread, followed by a slice of tomato, avocado slices then egg. Drizzle with sauce then sprinkle with chives if using.
Recipe adapted from Dairy Farmers of Canada, dairygoodness.ca