Heat 2 tablespoons of olive oil in a skillet over medium heat. Sauté 1 finely chopped onion for about 3-4 minutes until translucent.

Add 2 minced garlic cloves and 8 ounces of sliced mushrooms to the pan, cooking for 5-7 minutes until the mushrooms are tender and moisture evaporates.

Stir in a sprig of fresh thyme, along with salt and pepper to taste. Let the filling cool slightly.

On a floured surface, roll out a sheet of thawed puff pastry into a rectangle measuring approximately 12 by 16 inches.

Carefully transfer the rolled pastry to a parchment-lined baking sheet.

Make diagonal cuts along the sides of the pastry, 1 inch apart, leaving the center uncut for the filling.

Evenly spread the cooled mushroom mixture along the uncut center of the pastry, sprinkling 1 cup of shredded Gruyère cheese over it.

Fold the cut sides over the filling to create a braid-like effect and seal the ends tightly.

Brush the braid with a beaten egg and bake in a preheated oven at 400°F (200°C) for 25-30 minutes until golden brown.