• Preheat oven to 400℉.

Preparing the Mornay Sauce (Cheesy White Sauce)

• In a small saucepan set over medium heat, heat milk until just before it begins to boil. Turn off heat and set saucepan aside.
  1 ½ cups whole milk
• In a (separate) large sauce pan over medium-low heat, heat butter until it is fully melted and begins to foam.
  3 Tbsp unsalted butter
• Add in flour. Whisking constantly, cook butter and flour together for 1 to 2 minutes.
  3 Tbsp all-purpose flour
• Slowly pour hot milk into the flour/butter mixture. Whisking constantly, cook mixture until it forms a smooth sauce and then for another 3 to 5 minutes until it has thickened.
• Stir in salt, pepper, nutmeg, Gruyère, and Parmesan cheese. Cook until the sauce begins to bubble. Taste for seasonings. Remove sauce from heat and set aside.
  ½ tsp kosher salt

  ¼ tsp ground black pepper (or ground white pepper)

  ⅛ tsp ground nutmeg

  ⅓ cup Gruyère cheese, freshly grated

  ⅓ cup Parmesan cheese, freshly grated

Assembling and Baking the Sandwiches

• Meanwhile, place slices of bread (in a single layer) on a baking sheet lined with parchment paper. Bake for 5 minutes. Flip over and bake until nicely crisp and toasted (about 3 minutes).
• Remove toasted bread from the oven and set them on a cooling rack while assembling the sandwiches. (This will help prevent them from getting soggy.)
• On half of the toasted bread, spread Dijon mustard and top with sliced ham, followed by about ⅓ cup of the grated Gruyère cheese. Stack second slice of toast on top. Set each sandwich back onto the lined baking sheet.
• Spoon about ¼ cup of the cheesy white Mornay sauce over the top of each sandwich, spreading the sauce right to the edges of the top slice of toast. (This will help prevent the edges from burning in the oven!) Top sandwiches with remaining grated Gruyère and Parmesan cheeses.

  Pro tip: Depending on the size of your bread/sandwiches, there may be some Mornay sauce leftover, which is great for dipping! Otherwise, allow any extra to cool completely and store (in an airtight container) in the refrigerator or freezer for future use. (Mornay sauce can also be used for other great recipes, like a quick, homemade mac and cheese!)
• Bake for 8 minutes, then broil until cheese on top is bubbly and golden brown (about 3 to 5 minutes more). Serve warm.