Step 1: Preheat your oven to 400°F (200°C) and line two large baking sheets with parchment paper.
Step 2: Gently unroll one sheet of thawed puff pastry onto a lightly floured surface or directly onto one of the prepared baking sheets. Spread a thin, even layer of pesto over the entire surface, leaving a small border around the edges.
Step 3: Evenly sprinkle half of the shredded or diced fresh mozzarella over the pesto.
Step 4: Carefully unroll the second sheet of puff pastry and place it directly on top of the first, pressing down gently to seal the edges and encase the filling.
Step 5: Using a sharp knife or a pizza cutter, trim the edges of the layered pastry to create a neat rectangle. Then, cut the pastry lengthwise into strips, about 1 to 1.5 inches wide.
Step 6: Take each strip and gently twist it several times, starting from one end, to create a spiral.
Step 7: Form each twisted strip into a Christmas tree shape by gently shaping the twists into a triangular tree and folding the bottom up to create a ‘trunk’. Place the shaped trees onto the parchment-lined baking sheets, leaving a little space between each one.
Step 8: In a small bowl, whisk the egg yolk with a teaspoon of water to create an egg wash. Lightly brush the tops of each puff pastry tree with the egg wash. Sprinkle with a pinch of coarse sea salt, if desired.
Step 9: Bake for 12-15 minutes, or until the puff pastry is golden brown, puffed, and the cheese is bubbly. Keep an eye on them, as oven temperatures can vary. If they start to brown too quickly, you can loosely tent them with foil.
Step 10: Carefully remove the trees from the oven. If using, insert a small pretzel stick into the base of each tree after baking for an edible ‘trunk’. Serve warm and enjoy!