1. In a small pot on medium heat, melt 15 mL (1 tbsp.) butter.
2. Whisk in flour and cook for 2 minutes, then slowly whisk in milk. Simmer the sauce, whisking until thickened. Season with salt, pepper, and nutmeg. Put the lid on the sauce and remove from heat. Set aside.
3. Spread the remaining 30 mL (2 tbsp.) of butter on each slice of bread (one side only).
4. Place two slices of bread on a working surface, butter side down, and spread mustard on each. Top with 2–3 slices of ham, slices of tomato, slices of avocado, and 30 mL (1 oz.) grated cheese. Cover each with another slice of bread, butter side up.
5. Heat a large pan over medium heat and grill the sandwiches for about 3 minutes on each side or until golden brown. Transfer them to a non-stick baking pan. Set the oven to broil.
6. Using a spoon, top each sandwich with the sauce (it can drip down the sides a bit), then sprinkle with 15 g (1/2 oz.) of the remaining cheese. Broil the sandwiches for about 2 minutes or until the sauce starts to boil and darken.
7. Transfer each sandwich to a separate plate. Scrape the sauce from the baking pan and divide between the two plates. Sprinkle parsley on top and serve.

Chef’s secret

You can substitute any flavourful cheese, like Gouda or Jarslberg, for the Gruyere or Swiss.