First, preheat your oven to 375°F (190°C) so it’s ready when your braid is assembled. While it heats, warm the olive oil in a large skillet over medium heat. This initial step gets your kitchen smelling wonderful.
Add the chopped onion and minced garlic to the skillet. Cook them for about 3-4 minutes until they become soft and fragrant. Next, add the sliced mushrooms and dried thyme, cooking until the mushrooms are nicely browned.
Season the mushroom mixture with salt and pepper to your liking. Once cooked, remove the skillet from the heat and set it aside. It’s important to let the filling cool down before placing it on the pastry.
Lightly flour a clean surface and roll out your thawed puff pastry sheet. Using a knife, make diagonal cuts down both long sides of the pastry, leaving the center third intact. These cuts will become your braid.
Spoon the cooled mushroom mixture down the center of the pastry sheet. Sprinkle the shredded Gruyère cheese evenly over the top of the filling. This creates the delicious, cheesy core of your braid.
Now for the fun part! Fold the cut strips of pastry over the filling, alternating from one side to the other. This creates the beautiful braided pattern. Tuck in the ends to seal everything inside.
In a small bowl, beat the egg to create an egg wash. Use a pastry brush to gently coat the entire top surface of the braid. This step ensures a wonderfully golden and shiny crust after baking.
Carefully transfer the braid to a baking sheet lined with parchment paper. Bake for 25-30 minutes, or until the pastry is puffed up and a deep golden brown. Let it cool for a few minutes before slicing.