Steps
1. Preheat the oven to 200°C (400°F).
2. To prepare the béchamel sauce, melt butter in a small saucepan over medium heat.
3. Stir in the flour and cook for 1-2 minutes without browning.
4. Gradually whisk in the milk, stirring constantly until the sauce thickens.
5. Remove from heat, stir in Dijon mustard, salt, and pepper.
6. Lightly toast the bread slices.
7. Spread a thin layer of béchamel sauce on each slice.
8. Place artisan ham on one slice, then sprinkle a generous amount of Gruyère cheese.
9. Top with the second slice of bread, béchamel side down.
10. Spread béchamel sauce on top of the sandwich and sprinkle more Gruyère cheese.
11. Place the sandwich on a baking tray and bake for 10-12 minutes until golden and bubbly.
12. While the sandwich bakes, poach the egg: bring water to a gentle simmer, add a splash of vinegar.
13. Crack the egg into a small bowl and gently slide it into the simmering water.
14. Poach for about 3-4 minutes until the white is set but the yolk remains runny.
15. Remove the sandwich from the oven, carefully place the poached egg on top.
16. Garnish with chopped fresh chives if desired and serve immediately.