Pre heat your oven to 180°
Crack the egg and separate. Place the white in a small bowl and the yolk into a large mixing bowl.
To the mixing bowl add the minced steak, the oregano, the sage and the black pepper. Mix by hand to incorporate the ingredients and then form 4 thick square patties.
Slice the mushrooms and onion coarsely
Heat a large frying pan and sear the patties on each side for 1 minute. Remove them from the pan and place to one side.
Then cook the onions and mushrooms in the frying pan for 2 minutes. Add half a glass of water to the pan, then the garlic purée and the stock cube. Stir and cook until it starts to thicken then remove from the heat.
Unroll the pastry sheet and cut into four squares. Place a large spoonful of the mushroom and onion in the centre of each square, then place the patties on top at a 45 degree angle to the square.
Slice the cheese thinly and place on top of the patties, then gently pull each corner of the pastry over into the centre.
Place the en croutes onto a lined baking tray and brush with the egg white.
Bake for 30 minutes.
