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

In a skillet, heat 1 tablespoon of olive oil and 1 tablespoon of unsalted butter over medium heat. Add 1 diced yellow onion and sauté for about 5 minutes.

Add 8 ounces of sliced cremini mushrooms and 2 minced garlic cloves to the skillet. Cook for approximately 8 minutes until the mushrooms are browned.

Mix in 1 teaspoon of fresh thyme, along with salt and pepper to taste. Optionally, pour in 1/4 cup of dry white wine and cook for an additional 2-3 minutes.

Remove from heat and incorporate 4 ounces of cream cheese and 1/4 cup of grated Parmesan cheese into the mixture. Then fold in 1 cup of shredded Gruyère cheese until well combined.

Allow the mixture to cool slightly, then add 1 beaten egg to bind the filling together and stir well.

On a lightly floured surface, roll out 1 sheet of cold puff pastry into a rectangle approximately 10x14 inches and cut diagonal strips along the edges.

Spoon the cooled filling along the center of the puff pastry rectangle. Fold the diagonal strips of pastry over the filling, alternating sides to create a braid.

Carefully move the assembled braid to your lined baking tray and brush the surface with the remaining beaten egg.

Place the braid in the preheated oven and bake for 20-22 minutes until golden-brown.

Allow to cool for about 10 minutes before slicing.

Slice the braid gently and arrange on a platter for serving. Optional: garnish with freshly chopped parsley.