Whisk together the softened butter, garlic and parsley and set aside.
Cut slices in the baguette and use the implement of your choice to spread the garlic parsley butter into the gaps.
Slice the ham and cheese so it fits snugly into the baguette and add to each gap. Leave some cheese aside to sprinkle over the top.
Cut to fit on a baking tray (or place on a really long baking tray if you like).
Make the bechemal. In a small saucepan heat the butter to foaming, then whisk in flour and cook until it smells biscuity and nice. Incrementally pour in the milk, beating vigorously after each addition, to make a smooth sauce. Add the mustard and season.
Pour over the baguette and sprinkle with the remaining cheese, then bake until bubbling and melty and nice.