📋 Instructions

1. 1Heat the oven and toast — Heat your oven to 400°F (200°C). Lay the bread slices on a sheet pan and toast for 5 minutes, until dry and just barely colored at the edges, then flip and toast 2 minutes more. They should feel like firm crackers, not soft bread.
2. 2Build the light sauce — Melt the butter in a small saucepan over medium heat, whisk in the flour, and cook 1 minute until it smells toasty but hasn't browned. Pour in the milk in a steady stream, whisking constantly, and simmer 3 to 4 minutes until it thickens enough to coat a spoon.
3. 3Finish the béchamel — Pull the pan off the heat and stir in the yogurt, nutmeg, a pinch of salt, and half the Gruyère until smooth. Taste it; it should be savory and just tangy. If it looks gluey, whisk in a splash more milk.
4. 4Spread and layer — Butter one side of each toast slice, spread a thin layer of Dijon on half of them, then lay on the ham and a spoonful of sauce. Close the sandwiches and press gently so they hold together.
5. 5Top with cheese — Arrange the sandwiches on a parchment-lined sheet pan and spoon the remaining sauce over the tops, spreading it to the edges. Scatter the rest of the Gruyère and the Parmesan over everything.
6. 6Broil until bubbling — Broil on the middle rack at 500°F (260°C) for 3 to 4 minutes, until the cheese is melted, golden brown, and bubbling at the edges. Watch closely; you want deep gold, not dark brown.
7. 7Rest and cut — Let the tray sit 3 minutes so the cheese sets slightly, then cut each sandwich into 4 small squares with a serrated knife. Sprinkle with chives and serve warm.