Method
1. Pre-heat oven - 220°C / 425°F
2. Melt butter in a medium saucepan over medium heat until foamy
3. Add flour and cook, stirring, until mixture is pale and foamy, about 3 minutes
4. Gradually add milk, stirring until mixture is smooth
5. Cook, stirring, until sauce is thick and somewhat elastic, about 4 minutes
6. Remove from heat and whisk in mustard and nutmeg; season with salt
7. Spread bread slices with béchamel, dividing evenly and extending all the way to the edges
8. Place 4 slices of bread, béchamel side up, on a parchment-lined baking sheet; top with ham and half of cheese
9. Top with remaining slices of bread, béchamel side up, then top with remaining cheese and sprinkle with herbes de Provence
10. Bake until cheese is brown and bubbling, 10–15 minutes
11. Serve