Preheat the oven - Preheat the oven to 400°F (200°C) and line a baking sheet with parchment paper.

Thaw and roll out the puff pastry - Thaw the puff pastry according to the package instructions, then roll it out on a lightly floured surface to form a large rectangle (about 10x14 inches or 25x35 cm). Transfer the puff pastry to the prepared baking sheet. Be careful not to stretch the puff pastry too much as you roll it out, as this can cause it to become thin in some areas and thicker in others, which can affect the texture of the final product.

Prepare the filling - In a bowl, mix together the fresh spinach, crumbled feta cheese, chopped herbs, salt, and pepper until well combined.

Spread the filling on the puff pastry - Spread the spinach and feta mixture evenly over the puff pastry, leaving a small border around the edges.

Shape the tart into a tree - Using a sharp knife, make diagonal cuts on either side of the filling, starting at the top and working down to the bottom, to create a tree shape. Take care not to cut all the way through the pastry, as you want to keep the filling contained. Fold the cut edges of the pastry over the filling in a criss-cross pattern to create the appearance of tree branches. To create a more realistic tree shape, you can use a cookie cutter or a stencil to cut out a star or other shape at the top of the tree.

Brush with egg wash - In a small bowl, whisk together the egg with a splash of water to create an egg wash. Brush the egg wash over the exposed pastry using a pastry brush.

Bake the tart - Bake the tart in the preheated oven for 20-25 minutes, or until the pastry is golden brown and puffed up. Remove from the oven and let cool for a few minutes before slicing and serving.