Preheat the oven to 400°F (200°C). Line a large baking sheet with parchment paper or lightly spray it with cooking spray and set aside.

If using fresh spinach, wash thoroughly, remove tough stems, and dry. In a large skillet, heat 2 tablespoons olive oil over medium heat. Add the finely chopped onion and a pinch of salt, then sauté until translucent, about 4–5 minutes. Add the minced garlic and cook for 30–45 seconds until fragrant.

Add the spinach to the skillet in batches, letting each batch wilt before adding more. Cook until all the spinach is wilted and most of the moisture has evaporated, about 3–5 minutes. If using thawed frozen spinach, add it now and cook until heated through and moisture is reduced. Remove the pan from the heat and transfer the cooked spinach mixture to a fine-mesh sieve or bowl; press or squeeze out excess liquid and let cool slightly.

Finely chop the cooled spinach and transfer to a medium bowl. Add ricotta, grated Parmesan, shredded mozzarella, nutmeg, crushed red pepper flakes if using, a pinch of salt, and several grinds of black pepper. Mix until well combined and creamy. Taste and adjust seasoning; add a little lemon zest if desired.

Lightly flour your work surface. Unfold or roll the thawed puff pastry sheet into a rough rectangle about 12 x 14 inches, keeping it cold but pliable. Using a sharp knife or pizza cutter, slice the pastry lengthwise into 12 even strips for small trees or 8 wider strips for larger trees. Reserve any pastry scraps for cutouts.

Spread about 1 tablespoon of the spinach-cheese mixture down the center of each pastry strip, leaving a small margin along the edges. Use the back of a spoon to flatten and distribute the filling evenly.

Fold each strip over the filling so the filling is enclosed and the seam is on the bottom. With a sharp paring knife or kitchen scissors, make diagonal cuts from each side toward the center (but not cutting all the way through), creating branch-like slits along the length of the strip. Twist each cut segment slightly away from the center seam to create a layered, tree-like look. Shape the bottom into a short stump area where the skewer or pretzel will be inserted later.

Place the shaped trees on the prepared baking sheet, spaced about 1 inch apart. From the pastry scraps, cut small star shapes and place one star at the tip of each tree, pressing lightly to adhere. Insert a pretzel stick or a wooden skewer into the bottom of each tree to serve as the trunk, pushing it in just far enough so it’s steady but not exposed too far through the top.

In a small bowl, whisk the large egg with 1 tablespoon milk or water to make an egg wash. Brush the surface of each tree lightly with the egg wash. Sprinkle sesame or mixed seeds over the trees and a little extra grated Parmesan if desired.

Bake in the preheated oven for 15–20 minutes, or until the pastry is puffed and deep golden brown. Rotate the baking sheet halfway through baking if needed and tent with foil if edges brown too quickly.

Remove the baking sheet from the oven and let the pastries cool for 3–5 minutes on the pan so the cheese sets slightly. Transfer to a cooling rack or serving platter. Garnish with finely chopped fresh herbs if using and serve warm or at room temperature.

Store leftovers in an airtight container in the refrigerator for up to 3 days. Reheat in a 350°F (175°C) oven for 8–10 minutes to re-crisp the pastry before serving.