Prepare the sandwiches

• Butter the bread slices on one side and spread the mustard on the other side.
• Layer the ham, most of the Gruyère and the black truffle slices between two slices of bread.
• Close the sandwiches, buttered sides facing out.

Toast the croques

• Place the sandwiches in a hot frying pan, buttered side down.
• Cook until golden and crisp, then flip and cook the other side.
• Sprinkle the remaining Gruyère on top and let it melt.

Fry the eggs

• Fry the eggs in a little butter until the white is set and the yolk still runny.
• Season with salt and pepper.

Assemble and serve

• Top each croque with a fried egg.
• Finish with a few shavings of black truffle and serve immediately.