Boil the potatoes until soft.
Then mash together with mayonnaise, 20 milliliters milk, vinegar, salt and pepper until smooth.
Make cuts on one puff pastry sheet, creating a mesh pattern.
Season beef round with salt and pepper.
Grease frying pan with olive oil and cook beef until lightly browned on all sides.
Take out the beef to set aside and reserve the pan drippings.
Combine all the ingredients for the sauce with the pan drippings.
Cook until smooth and slightly thickened.
Place beef onto the puff pastry followed by the mashed potatoes.
Wrap everything up with the puff pastry.
Top with the second sheet of puff pastry and cut off the excess with a knife.
Combine egg and 20 milliliters milk to make an egg wash.
Brush egg wash onto the pastry and bake for 30 minutes at 220 degrees C.
Serve with sauce and garnish with rosemary.