Finely chop enough button mushrooms to fill one cup. You can also use a food processor to get it very small.
Finely slice 2 medium onions
Melt 2 tablespoons butter and sauté the mushrooms for about 5 minutes until they soften, then add the onions.
Turn the heat down and slowly sauté the onions. This should take at least an hour. You can stir it periodically. You can add another tablespoon of butter if needed.
While the onions cook, grate one cup of cheese.
Melt 2 tablespoons of butter with 3 tablespoons of marmite. Mix it into the onion and mushroom mixture.
Cook until the onions become thick and the marmite mixture has fully cooked down.
Once the onions are ready, put the mushroom and onion jam in a bowl and let it cool completely before putting it in the fridge.
Once the jam is cold, mix it with the cup of grated cheese.
Let the puff pastry come to room temperature and set your oven to 350 degrees.
Throw some flour on your work surface and roll the pastry sheets slightly.
Now use a cup or glass with a 3 inch diameter. Press out circles on the sheets until you have 36, 12 on a sheet.
Prepare a large cookie sheet with parchment paper.
Put half of the circles on the cookie sheet. Put one large teaspoon of the jam on each and then cover with the remaining circles. By using a fork, press down around the sides of each pie until it is closed.
Mix one egg yolk and a little bit of water. Brush the egg wash over the tops and sides of the pies.
Cut a small cross with a knife in the middle of each pie so that the steam can come out so that the pie does not get soggy.
When done, place it in the middle rack of the oven and cook till golden brown-20 to 25 minutes.
Then cool and enjoy with a nice glass of wine!