Roll the pastry out so its around 40 centimeters wide and 1/2 centimeters thick. You want the pastry to look a big like a landscape piece of A3 paper. then cut in half lengthways. Layer over the half of the mustard, follow by half the ham, half the cheddar, half the onions, half the sliced mozzarella and dot over some slices of tomato, not too many as you don't want to make the pastry soggy.
Repeat with the other half of the pastry and ingredients. roll the party up from the top, towards you and then cut evenly into 6 thick slices. Repeat withe the other pastry. Turn the rolls cut side down and layer into a tight fitting oven tray, that is lightly greased. Brush oven the melted butter and cover the tray in tin foil.
Place in the oven for 20 minutes and then remove the tin dil and cook for a further 10-12 minutes until the pastry is golden. Tare into individual rolls and serve.