• If your puff pastry is frozen, thaw it following the package instructions and then make sure it is at refrigerator temperature before working with the dough. If it is at room temperature it will be too soft to work with.
  
• Preheat your oven to 400F

Duxelles

• Rinse the button mushrooms under cold water to clean them of any excess dirt. If there is any dirt attached to the stems, cut that part away.
  
• Slice the mushrooms thinly, caps and stems.
  
• Finely chop the sliced mushrooms until you reach a gravel-y consistency and set them aside
  
• Finely mince 1.5 cups of onion and set aside.
  
• Finely chop 1.5 Tbsp garlic and set aside.
  
• Pick 1 Tbsp of fresh thyme leaves and set aside.
  
• In a large saute pan or dutch oven, melt 2 Tbsp butter over medium-high heat.
  
• When the butter is melted, add the cremini mushrooms, minced onions, minced garlic, and a generous pinch of salt.
  
• Cook on medium-high heat while stirring until the mushrooms begin to release their liquid.
  
• Once the mushrooms have released their liquid, turn the heat down to medium and allow everything to cook for about 15- 20 minutes, stirring occasionally, until the liquid has evaporated.
  
• Once the liquid has evaporated, cook the mushrooms for an additional 2-3 minutes so that the mushrooms start to brown very slightly.
  
• Add 1/3 cup madeira (or other cooking wine) to the pan and cook until it has evaporated.
  
• Add 1/4 cup heavy cream and 1 tbsp picked thyme leaves to the pot.
  
• Cook this mixture over medium heat until the cream has thickened and reduced.
  
• Once the cream has reduced and the mixture is no longer liquidy, turn off the heat and season the mixture with 2 tsp vinegar and salt and pepper to taste. The mixture should be pleasantly seasoned. Set this mixture in a bowl and place in the refrigerator until it has come down to at least room temperature before using on the tart.
  

Toppings

• Prepare the taleggio by cutting away any paper or rind so you are left with only the soft interior.
  
• Weigh out about 3 oz of taleggio (about 1/3 cup) and use your fingers to form it into small balls about 1/2 inch in diameter. Set the balls on a plate and set them aside. If you are using a harder cheese, simply cut it into approximately 1/2 inch cubes.
  
• These are the mushrooms that I used, beech on the left and maitake on the right. Clean the mushrooms if necessary.
  
• For the maitake mushrooms, cut away the woody base of the mushroom and use your fingers to tear the mushrooms into approximately 1 inch pieces.
  
• The beech mushrooms simply need to have the woody base cut away so the individual mushrooms will fall apart. Set both types of mushrooms in a mixing bowl.
  
• Add 1.5 Tbsp olive oil to the mushrooms immediately toss the mushrooms so that the oil lightly coats all the mushrooms. The mushrooms will want to soak up the oil quickly so toss them as soon as the oil hits the mushrooms. If necessary, add another splash of oil so that all the mushrooms have a light coating. Season the mushrooms with salt and pepper. Set aside.
  
• Beat the egg with aproximatly 4 Tbsp water and set aside.
  

Making the tart

• Make sure your oven has reached 400F before working with the puff pastry. Open up the thawed, chilled package of puff pastry onto a lightly floured countertop. Once the dough is out of the fridge, try not to let it sit around and get warm or else the dough will get soft and hard to work with. Try to work quickly and, if necessary, throw the dough back in the fridge until it is cold enough to work with.
  
• If the pastry is folded, open it up and lightly flour the top and bottom.
  
• Use the rolling pin to roll out the dough just until the 11 or 12 inch plate will fit within the rectangle of dough. Place the plate upside down on the cold dough.
  
• Use a sharp knife to cut around the plate and yield a round of puff pastry. Save dough scraps for another use.
  
• Quickly transfer the dough round onto a baking sheet lined with parchment paper (you could get away with aluminum foil that is lightly oiled), Use a fork to prick holes all over the dough, spaced about 1 inch apart, leaving a 1/2 inch rim unpricked around the edges. Spread all of the duxelles evenly on the puff pastry, again, leaving a 1/2 inch rim on the edges.
  
• Scatter the oiled/seasoned beech and maitake mushrooms evenly on top of the duxelles.
  
• Use a brush or your finger to brush the exposed 1/2 inch perimeter of dough with a thin coating of egg wash. This will help the "crust" get nice color. Put the baking sheet into the 400F oven and bake the tart for about 30-35 minutes. Do not open the oven for the first 20 minutes.
  
• When the tart has puffed, and the crust has become golden, as shown. Remove the tart from the oven.
  
• Quickly scatter the taleggio all over the tart and immediately throw it back in the oven for 5 minutes, or until the cheese has completely melted.
  
• After the cheese has melted, remove the tart from the oven and if possible, carefully transfer it to a wire rack. Allow the tart to cool for 10 minutes.
  
• Top the tart with as much freshly grated parmesan as you like and sprinkle on the chives. Cut into 8 to 10 pieces. Tart can be enjoyed warm or at room temperature. Leftover tart can also be refrigerated and reheated in a 350F oven if desired.