Lamb and haggis Wellington

1 skinless chicken breast

500g of lamb cannons, in a single piece

200g of haggis, crumbled

1 egg white

salt

pepper

100g of plain flour

2 tbsp of mustard

2 sheets of puff pastry

1 dash of double cream

3 eggs

300ml of semi-skimmed milk

1 tbsp of sunflower oil, plus extra for frying