Step by Step method

1. Step 1.

    

2. Step 2.

   For the sandwiches: Adjust oven rack 6 inches from broiler element and heat oven to 375 degrees. Line rimmed baking sheet with aluminum foil and spray with vegetable oil spray.

3. Step 3.

    

4. Step 4.

   Brush bread on both sides with melted butter and place on prepared sheet. Bake until light golden brown on top, about 10 minutes. Remove sheet from oven and flip slices. Return to oven and bake until golden brown on second side, about 3 minutes. Reserve 4 slices for sandwich tops; evenly space remaining 4 slices on sheet.

5. Step 5.

    

6. Step 6.

   For the Mornay sauce: Melt butter in small saucepan over medium heat. Whisk in flour and cook for 1 minute. Slowly whisk in milk and bring to boil. Once boiling, remove from heat and quickly whisk in Gruyère, Parmesan, salt, pepper, and nutmeg until smooth.

7. Step 7.

    

8. Step 8.

   Spread 1 tablespoon Mornay on each slice of toast on sheet. Then, folding ham slices over themselves multiple times so they bunch up, divide ham evenly among slices of toast. Spread 2 tablespoons Mornay on 1 side of each reserved slice of toast and place slices Mornay side down on top of ham.

9. Step 9.

    

10. Step 10.

    Spread 2 tablespoons Mornay evenly over top of each sandwich, making sure to completely cover toast, including edges (exposed edges can burn under broiler). Sprinkle sandwiches with Parmesan, followed by Gruyère.

11. Step 11.

     

12. Step 12.

    Bake until cheese on top of sandwiches is melted, about 5 minutes. Turn on broiler and broil until cheese bubbles across tops of sandwiches and edges are spotty brown, about 5 minutes. Serve.