BÉCHAMEL SAUCE: Heat the butter in a saucepan over a gentle heat until just melted. Stir in the flour with a wooden spoon. Add the milk in three lots, stirring continuously, until well incorporated. Simmer for 5 minutes, whisking while cooking, until thick and silky.

Add the parmesan, nutmeg and mustard and stir to combine. Season to taste with salt and pepper, then refrigerate to cool.

Preheat the oven to 200°C regular bake and put an oven tray in to heat.

to assemble: Butter one side of each piece of bread. Lay 2 pieces buttered side down on a piece of baking paper. Spread each piece with 3 tablespoons of béchamel sauce and top with 3 pieces of ham. Top with the onion (if using) and remaining bread, buttered side up. Spread with 3 more tablespoons of béchamel each. Sprinkle with gruyère and pop in the oven until golden and bubbling, about 15 minutes. Top each sandwich with a fried egg.

Caramelised Onions 

INGREDIENTS
2 tablespoons olive oil
1 tablespoon butter
3 large brown onions, sliced 1cm thick
2 cloves garlic, crushed
2 teaspoons finely
chopped thyme
sea salt and ground pepper
1 tablespoon brown sugar
2 tablespoons balsamic vinegar

METHOD

Heat the oil and butter in a frying pan and add the onions, garlic and thyme. Season generously and cook for 20 minutes, stirring occasionally, until softened. Add the sugar and vinegar and continue to cook until deeply golden and glossy.