Put ground beef into a mixing bowl.
Add chopped parsley, onion to the ground beef and mix together.
Add the egg yolk and mix well.
Break mix into 4 equal sized balls and on a flat surface pat the balls into round flat circles.
Keep in freezer for atleast 12 hrs in order to make it firm.
Preheat a little butter in a pan.
Gently fry burgers on both sides until cooked through.
Cut the burger from middle and toast the inside.
Spread a little butter/mayonise/ketchup on inside of the bun.
While warm add the burger.
:- Serve with french fries and salad.