Preheat the oven to 180 degrees/160 fan.
Unroll the pastry and place the mustard in a strip down the middle, making the middle third yellow.
Squish the sausage meat into a long sausage and place in the middle of the pastry on top of the mustard.
Add the cut up shallots and onions.
Season with salt, pepper and some dried oregano/thyme too if you would like.
Fold one side of the pastry over the sausage meat on to the middle third and do the same with the other third.
Cut up the roll in to good inch slices. I normally get about 12-14 out of a roll.
Put the slices on to a non stick baking tray or one with greaseproof paper. Don't put them too close together as they will spread.
Brush with the egg and bake in the oven for 30 minutes until golden brown.
Serve when hot from the oven or let them go cold to take them for a picnic.