Step 1

Preheat the oven to 375°F / 190°C / Gas Mark 5, no fan (if you have a convection oven reduce the temperature by about 20° or follow the manufacturer's instructions).

Step 2

Cut the button mushrooms into 1/4-1/3 inch (5-7mm) slices. Chanterelle mushrooms: leave small mushrooms whole, cut medium mushrooms into 2 parts, and large mushrooms into 3-4 parts. Don't cut the mushroom too thinly.

Step 3

Cut the shallot into slices, finely chop the garlic.

Step 4

Heat butter in a large frying pan over medium-low heat. When hot add the onions and thyme, cook for a couple of minutes, until the onion is soft. Add the garlic and cook for 30 seconds, stirring.

Step 5

Transfer the onions with garlic to a plate.

Step 6

Add oil to the pan and increase the heat to the highest setting. Add mushrooms in an even layer, starting with button mushrooms (chanterelle mushrooms should be on top of button mushrooms). Cook without stirring for 1-2 minutes then add the vinegar and cook, stirring, for about 4-8 minutes (depending on how powerful your stovetop is) until the mushrooms have reduced their volume in half and all the liquid is evaporated. Don't stir the mushrooms for about a minute when almost all liquid is evaporated, letting the mushrooms brown a little.

Step 7

Add the onions with garlic back to the pan, stir with the mushrooms. Season the mushrooms and onions with salt and pepper. Transfer to a plate and leave to cool slightly.

Step 8

Take the puff pastry out of the fridge. With a tip of a sharp knife, score through the top layers of the puff pastry by running the knife along all four sides of the dough (leaving about 1-inch / 2.5 cm border). Do not cut all the way through the dough. Transfer the pastry, along with the parchment paper that comes with it to a baking sheet. Prick the dough (but not the borders) with a fork.

Step 9

Sprinkle the cheese in an even layer over the dough.

Step 10

Top with cooled mushrooms and onions.

Step 11

Brush the edges of the tart with beaten egg. Prick the dough with a fork a couple times more (or it will puff up in the oven), not the edges, just the center of the tart.

Step 12

Bake the tart for about 20-25 minutes or until golden.

Step 13

Enjoy!