Follow these detailed steps carefully to ensure your pinwheels are perfectly cooked, with a crispy exterior and a warm, creamy center. The process can be broken down into two main stages: creating the savory filling and then assembling and baking the pinwheels.

Part 1: Preparing the Savory Filling

1. Prepare the Spinach: The most crucial step for success is preparing your spinach. Take your thawed package of chopped spinach and place it in a clean kitchen towel or several layers of heavy-duty paper towels. Gather the towel around the spinach and wring it out over the sink, squeezing as hard as you can until no more liquid comes out. You will be amazed at how much water is released. This step is non-negotiable for preventing a soggy bottom. Set the very dry spinach aside.
2. Sauté the Aromatics: Heat the olive oil in a large skillet or pan over medium heat. Once the oil is shimmering, add the finely diced yellow onion and sauté for 3-4 minutes, or until it becomes soft and translucent. Add the minced garlic and cook for another minute until fragrant, being careful not to let it burn.
3. Cook the Mushrooms: Add the finely chopped cremini mushrooms to the skillet with the onions and garlic. Increase the heat slightly to medium-high. Cook the mushrooms, stirring occasionally, for about 5-7 minutes. The mushrooms will first release their water; continue cooking until all that liquid has evaporated and the mushrooms are browned and slightly caramelized. This process concentrates their flavor.
4. Combine the Filling: Remove the skillet from the heat and let the mushroom mixture cool for a few minutes. In a large mixing bowl, combine the softened cream cheese, grated Parmesan cheese, shredded mozzarella, dried oregano, salt, and black pepper. Mix with a spatula until well combined.
5. Fold in Vegetables: Add the cooled mushroom mixture and the super-dry squeezed spinach to the cream cheese mixture. Stir until everything is evenly incorporated. Taste the filling and adjust the seasoning with more salt or pepper if needed. Your delicious, savory pinwheel filling is now ready.

Part 2: Assembling and Baking the Pinwheels

1. Preheat and Prepare: Preheat your oven to 400°F (200°C). Line a large baking sheet with parchment paper to prevent the pinwheels from sticking and make cleanup a breeze.
2. Roll out the Pastry: Gently unfold your thawed puff pastry sheet onto a lightly floured surface. If needed, use a rolling pin to lightly roll it out into a rectangle of approximately 10×15 inches, smoothing out any creases.
3. Spread the Filling: Using an offset spatula or the back of a spoon, spread the spinach and mushroom filling evenly over the entire surface of the puff pastry, leaving a small half-inch border along one of the long edges. This bare edge will help seal the roll.
4. Roll it Up: Starting from the long edge opposite the bare border, carefully and tightly roll the puff pastry into a log. The tighter you roll, the more defined the spiral will be in your finished pinwheels. When you reach the end, lightly moisten the bare edge with a little water or some of the egg wash and press it firmly to seal the log shut.
5. Chill the Log (Optional but Recommended): For the cleanest cuts, wrap the log tightly in plastic wrap and place it in the freezer for 15-20 minutes or in the refrigerator for 30 minutes. This will firm up the pastry and the filling, making it much easier to slice without squishing.
6. Slice the Pinwheels: Remove the chilled log from the freezer or refrigerator. Using a very sharp serrated knife, gently slice the log into individual pinwheels, each about 1/2 to 3/4-inch thick. You should get approximately 18-20 pinwheels.
7. Arrange and Bake: Place the sliced pinwheels cut-side up on the prepared baking sheet, leaving about 2 inches of space between each one to allow them to puff up and cook evenly. Brush the tops and sides of each pinwheel with the prepared egg wash. This will give them a beautiful shine and a deep golden color.
8. Bake to Perfection: Place the baking sheet in the preheated oven and bake for 18-22 minutes, or until the pastry is puffed, golden brown, and crisp.
9. Cool and Serve: Remove the pinwheels from the oven and let them cool on the baking sheet for a few minutes to set before transferring them to a wire rack. They can be served warm or at room temperature.