Basically fancy French grilled cheese! It's really quick to make and can serve a lot of people if you use a long loaf.
From Martha Stewart Living, Nov. 2011
1 1/4 cups whole milk (I used half & half)
2 tbsp unsalted butter
3 tbsp flour
3 tbsp fresh thyme leaves, divided (I used rosemary)
Coarse salt & freshly ground pepper
1 long loaf rustic bread, such as Ciabatta (I used TJ's Rustic Loaf)
3 tbsp Dijon mustard
3/4 lb sliced ham
4 cups grated Gruyere cheese (8 oz)
1 cup finely grated Parmesan cheese
1. Preheat oven to 400. Pour the milk into a saucepan and heat over medium for about 5 minutes. In another saucepan, melt the butter over medium. When it's melted, add flour and whisk occasionally for 2 minutes. Add 1 1/2 tbsp of herbs, then carefully mix in milk. Simmer bechamel for 2 minutes, whisking. It will be thick. Remove from heat and add 1 tsp salt and 1/2 tsp pepper.
2. Cut the top crust off the loaf of bread. Slice the loaf horizontally. Evenly layer the bottom half with mustard, ham and gruyere. Place both halves side by side on a baking sheet and toast until all the cheese is melted.
3. Remove from oven. Assemble the loaf and spread the bechamel on top and sprinkle on remaining cheese and herbs. If you have a broiler, use it, if not keep it at 400. Bake until golden brown and bubbling, 6-7 minutes. Remove from oven and let sit for 10 minutes before cutting into slices.