Prepare the tray

• Preheat the oven to 200 °C.
• Brush the baking tray with melted butter and line it with baking paper.

Build the upside-down layers

• Lay the ham slices on the tray, overlapping slightly.
• Scatter the grated Gruyère over the ham.
• Brush the pastry with mustard and place it mustard-side down on the cheese, pressing gently.

Seal and egg wash

• Prick the pastry all over with a fork.
• Brush the top with beaten egg.

Bake

• Bake until puffed and deep golden.

Flip and serve

• Rest for a few minutes, then invert onto a board so the ham and cheese are on top.
• Cut into portions and serve warm.