Preheat your oven to 365 deg F (185 deg C). Line two 9x13 baking sheets with parchment paper.
In a large skillet, over medium low heat, cook your diced pancetta. Cooking at a low heat will allow it to render nicely. Once crispy, remove your crispy pieces to a cutting board (leaving the rendered fat in the pan) and dice more finely. Set aside.
Add your olive oil and onion to the same skillet. Cook over medium high heat until the onion is translucent.
Add your mushrooms to your pan and stir them in until cooked through. You do not need to remove all the moisture from the mushrooms, just cook them briefly - 1-2 minutes. Remove pan from heat.
Add in mayonnaise, salt, black pepper, garlic, parmesan, and mozzarella, and stir to combine.
On a lightly floured surface, roll out each of your 2 sheets of puff pastry to about 12 x 16 inches.
Divide your mushroom filling in two, and, working with one sheet at a time, lay your filling out onto your sheet.
Roll the sheet tightly, starting from the longer side. Cut each roll into 1 inch slices, and lay these pinwheels flat onto your prepared baking sheets, at least 2 inches apart.
Bake 25-30 minutes, or until puffy and golden brown. Serve warm, and enjoy!