Photo: Michael Brewer (CC BY-SA 2.5) via Wikimedia Commons · source
The Paris café sandwich: ham and Gruyère in buttered pain de mie, topped with a little béchamel and more cheese and baked until it bubbles and browns. Add a fried egg on top and it's a croque-madame.
4 slices pain de mie or thick white sandwich bread
4 slices good cooked ham
120 g (1 1/4 cups) grated Gruyère or Comté
2 tbsp soft butter
1 tsp Dijon mustard
For the béchamel: 15 g (1 tbsp) butter, 1 tbsp flour, 180 ml (3/4 cup) whole milk, a pinch of nutmeg, salt and white pepper
Optional: 2 eggs for a croque-madame
How to make it
Béchamel (6 min): Melt the butter, stir in the flour and cook 1 minute without browning. Whisk in the milk gradually and simmer, whisking, until thick like custard, 3 to 4 minutes. Season with nutmeg, salt and pepper and stir in 2 tbsp of the cheese.
Heat (2 min): Heat the grill (broiler) high, with a rack about 15 cm (6 in) below.
Build (4 min): Butter one side of each slice. Lay 2 slices butter side down on a tray, spread thinly with mustard and a little béchamel, add ham and a third of the cheese, then top with the other slices, butter side up.
Toast (4 min): Toast in a dry skillet over medium heat 2 minutes a side until golden.
Top and grill (4 min): Spread the rest of the béchamel on top, scatter the remaining cheese, and grill until bubbling and spotted brown, 3 to 4 minutes. Top with a fried egg for a madame. Serve with a green salad.
Tips
Gruyère or Comté, not cheddar; you want nutty and stretchy.
Watch the grill closely in the last minute; cheese goes from brown to burnt fast.