Preheat your oven to 350°F (180°C) and line a baking sheet with parchment paper.

Melt 2 tablespoons of unsalted butter and 1 tablespoon of olive oil in a skillet over medium heat. Add 1 diced yellow onion and sauté for about 5 minutes until soft.

Stir in 2 minced garlic cloves and 8 ounces of chopped cremini mushrooms. Sauté for approximately 8 minutes until golden brown. Season with fresh thyme, salt, and black pepper.

Reduce heat to low, mix in 4 ounces of cream cheese, 1/4 cup of grated Parmesan cheese, and 1 teaspoon of Dijon mustard. Stir until creamy, then fold in 1 cup of shredded Gruyère cheese.

Roll out 1 sheet of puff pastry into a rectangle, creating 8 diagonal slits on each side for braiding.

Spread the mushroom filling along the center and fold the pastry sides over, alternating the slits to braid. Press seams to seal.

Transfer to the baking sheet. Brush with a beaten egg and bake for 20-22 minutes until puffed and golden brown.

Let cool for about 10 minutes and garnish with fresh parsley before serving.