Preheat oven to 425F
If sausage is in casing, remove from casings into a bowl.
Add bread crumbs and mustard to the sausage.
Roll out puff pastry until ¼" thick rectangle roughly 12" by 18". Cut it in half to have two 6" by 18" strips.
Divide the meat in half and make a log down the centre of the pastry. (This is where its very helpful have watched the video).
Brush the egg down the seam where you cut so it gets a good inch of egg wash on each piece of pastry.
Now fold the ugly side of the pastry (the uncut) over the meat, then the egg wash side. make sure its overlapping so it doesn't bust out. Gently press it to make sure it seals and flatten it some.
Flip it seam side down onto a parchment lined tray and gently press to make it less round more oval.
Cut to desired size, 1 inch for sausage roll bites of 3-4 inches for full size.
Brush with remaining egg wash and if desired, sprinkle poppy or sesame seeds on top.
Spread rolls out so they have space to bake.
Bake 15- 20 minutes, or until puffy and browned.
There will be fat/butter on the tray but don't worry about it. Let cool 5-10 minutes on the tray, then transfer to cooling racks. Store in fridge, eat cold or heated up!