Begin by preheating your oven to 350°F (180°C) and prepare a baking tray by lining it with parchment paper.

In a large skillet, heat 2 tablespoons of olive oil and 2 tablespoons of unsalted butter over medium heat. Once the butter melts and bubbles, add 1 diced yellow onion and sauté for approximately 5 minutes until it softens and turns translucent.

Add 3 cups of sliced cremini mushrooms to the skillet, along with 2 minced garlic cloves. Stir well and cook for about 8 minutes until the mushrooms have browned and released their moisture.

Sprinkle in 1 tablespoon of fresh thyme, along with salt and black pepper to taste. If using, pour in ¼ cup of dry white wine and cook until the liquid is mostly evaporated.

Stir in 4 ounces of cream cheese and ¼ cup of grated Parmesan until melted and thoroughly mixed. Fold in 1 cup of shredded Gruyère cheese until it’s just combined.

Once the filling has cooled slightly, add 1 beaten egg to the mixture and stir until it binds together.

Unroll one sheet of cold puff pastry onto a floured surface. Carefully cut diagonal strips along each side, leaving a center section wide enough to hold the filling.

Spread the cooled filling down the center of the puff pastry, fold the ends over the filling to encase it, and braid the strips by alternating them from side to side.

Transfer the assembled braid onto your parchment-lined baking tray. Brush with the remaining beaten egg.

Bake for 20-22 minutes, or until the pastry is puffed and golden brown.

Let the braid cool for about 10 minutes on the tray before slicing.

Slice the puff pastry braid into portions and arrange them on a serving platter. Garnish with chopped fresh parsley if desired.