In a large skillet, melt the butter over medium heat.

Add the sliced onions and garlic. Sauté for 10–12 minutes, until soft and lightly caramelized.

Add the mushrooms, salt, pepper, and thyme. Cook for another eight to 10 minutes, stirring occasionally, until the mushrooms are browned and tender.

Deglaze the pan with the Marsala wine. Let it cook down for about one to two minutes.

Add the spinach and parsley. Stir until spinach is wilted and well combined. Remove from heat and let cool slightly.

Preheat oven to 400 degrees Fahrenheit (200 degrees Celsius).

On a lightly floured surface, roll out the puff pastry into a large circle about 1/8 inch thick. Transfer to a parchment-lined baking sheet.

Spread the Tuscanini pesto generously over the center, leaving a four-inch border around the edges.

Spoon the mushroom mixture evenly over the pesto.

Fold the edges of the dough over the filling, pleating as you go to form a rustic border. The center should remain exposed.

Brush the folded edges with the beaten egg.

Top the entire galette with shaved or grated mozzarella and a bit of chopped parsley.

Bake for 35–40 minutes, or until the crust is golden brown and crisp.

Remove from the oven and sprinkle with additional mozzarella, if desired.

Let cool slightly before slicing.

Serve warm with a side of extra red pesto for dipping. Perfect as a light dinner, appetizer, or weekend brunch centerpiece.