Preheat the grill to medium-high. Line the grill tray with foil. Put the butter in a saucepan over a medium-low heat to melt it. Brush one side of each slice of bread liberally with melted butter and put under the grill butter-side uppermost until golden and crisp, then set aside.
Stir the flour into the remaining butter to make a paste, cook for a minute, then gradually whisk in the milk, a little at a time, until smooth. Simmer for a few minutes, until thickened, then take off the heat and stir in 60g cheese until melted. Grate in a little nutmeg, stir and season lightly.
Spread the untoasted sides of four slices of bread with mustard, then put the ham on top, followed by the cheese, and pop under the grill for a couple of minutes until the cheese has melted.
Top with the rest of the bread, with the toasted side uppermost, and push down, then put the sandwiches on to the grill tray and spoon on the bechamel sauce. Grill for about 5 minutes, until golden and bubbling. Serve immediately with the salad.
To make the salad: combine the carrot, beetroot, gherkins, onion and radishes in a bowl. In a separate bowl whisk together to olive oil, lemon juice and season with salt and black pepper. Stir this through the vegetables. Sprinkle over the toasted sesame seeds.