Ingredients:

  • 3 Tbsp Unsalted Butter (45g), divided
  • 3 Tbsp All-Purpose (Plain) Flour (45g)
  • 1 ½ cups Whole Milk, warmed (360 ml)
  • ¼ tsp Freshly Grated Nutmeg
  • Salt and Black Pepper, to taste
  • ½ cup (50g) Freshly Grated Gruyère Cheese (for sauce)
  • 8 slices Thick-cut Sliced Bread (Sourdough or Brioche)
  • 2 Tbsp Dijon Mustard (30 ml)
  • 8 slices High-quality Thinly Sliced Ham (115g)
  • 1 cup (100g) Freshly Grated Gruyère Cheese (for topping)

Instructions:

  1. Preheat the oven to 400°F (200°C). Lightly butter the 9x13 inch baking dish.
  2. Grate all the Gruyère cheese, separating the ½ cup portion (50g) for the sauce from the 1 cup portion (100g) for the topping.
  3. Toast the bread slices for 2-3 minutes per side until just barely golden. This optional step helps prevent the bread from turning soggy.
  4. Make the Roux: Melt 3 Tbsp of butter in a saucepan over medium heat. Whisk in the 3 Tbsp of flour immediately. Cook for 1–2 minutes, stirring continuously, until nutty but not browned (a blonde roux).
  5. Add Milk: Gradually whisk in the warm milk. Once all the milk is incorporated, bring the sauce to a gentle simmer, whisking constantly until it thickens sufficiently to coat the back of a spoon (about 5–7 minutes).
  6. Season and Enrich: Remove from heat. Stir in salt, pepper, and nutmeg. Add the ½ cup of Gruyère reserved for the sauce and stir until melted and smooth (Mornay sauce). Reserve ¼ cup of this Mornay sauce for drizzling inside the sandwiches.
  7. Coat the Bread: Spread a thin, even layer of Dijon mustard on one side of each bread slice.
  8. Assemble the Filling: Take four slices of bread (mustard side up) and spread a teaspoon of the reserved Mornay sauce on top. Layer with 2 slices of ham and a scattering of grated Gruyère. Top with the remaining four slices of bread (mustard side facing the ham).
  9. Arrange and Sauce: Place the assembled sandwiches snugly into the prepared baking dish. Pour the remaining large batch of Mornay sauce evenly over the tops of all four sandwiches, ensuring all exposed bread is covered edge-to-edge.
  10. Final Topping: Sprinkle the remaining 1 cup (100g) of grated Gruyère evenly over the top of the sauced sandwiches.
  11. Bake: Bake for 20–25 minutes, or until the sauce is deeply bubbly, the cheese topping is melted and golden brown, and the bread edges are crisp.
  12. Rest and Serve: Allow the Croque-Monsieur to rest in the pan for 5 minutes before serving to let the sauce firm up slightly.