In a medium saucepan, melt butter over low heat. Whisk in flour and cook for 2 minutes. Gradually add milk, cook until thickened, then season with nutmeg and mix in Dijon mustard.

Spread a thin layer of butter on one side of each bread slice. Preheat a large skillet over medium heat.

Lay 4 slices of bread butter-side down in the skillet. Add a layer of ham and sprinkle grated gruyère on top. Pour béchamel sauce over cheese and top with another slice of bread.

Cook for 3-4 minutes until golden brown, flip and cook the other side for 3-4 minutes.

Transfer to a baking sheet and broil for 2-3 minutes until bubbly and golden.

Remove from oven, let cool for a minute, slice in half and serve hot.