1-Getting Started: First, preheat your oven to 350 degrees F (180 degrees C) and line a baking tray with parchment paper for easy cleanup. Heat olive oil in a skillet over medium-high heat and cook the finely diced onion until soft, about 5 minutes, to build a flavorful base.

2-Getting Started: Next, add the cleaned and sliced mushrooms along with minced garlic, cooking until the mushrooms brown, which takes about 5 to 8 minutes. Reduce the heat to low and mix in the softened cream cheese, finely grated Parmesan cheese, dried oregano, salt, black pepper, and fresh chopped parsley until creamy.

3-Assembling and Baking: Remove the mixture from heat and stir in one whisked egg until fully incorporated for a rich filling. Unroll the ready-to-roll frozen puff pastry sheet on the parchment paper and make cuts as described: cut into thirds, then create about eight diagonal cuts along each side, plus cuts for the top corners and an oval shape.

4-Assembling and Baking: Spread the mushroom filling down the center, leaving space at the top and bottom. Fold the pastry strips over in a crisscross pattern and press the edges to seal. Brush with the remaining whisked egg for a golden finish, then bake for 20 to 22 minutes until puffed and golden. Let it cool for 10 minutes before serving hot, as this step helps the flavors blend.