Prepare the Filling: In a medium bowl, combine the softened butter, granulated sugar, lemon zest, lemon juice, and poppy seeds. Use a fork or a small whisk to mix everything really well until it's smooth and creamy.

Prep the Puff Pastry: Lightly flour your work surface. Unfold one sheet of the thawed puff pastry. Gently roll it out just a little bit, to about a 10x12 inch rectangle. This helps to even it out and make it a bit thinner.

Spread the Filling: Spread half of the lemon-poppyseed mixture evenly over the puff pastry sheet. Make sure to get it all the way to the edges.

Cut into Triangles: Using a pizza cutter or a sharp knife, cut the pastry sheet into triangles. I usually cut it in half lengthwise first, then cut each half into 3 or 4 triangles. You should get about 6-8 triangles per sheet.

Roll into Croissants: Starting at the wide end of each triangle, gently roll it up towards the point. Place the rolled croissants onto a baking sheet lined with parchment paper. Make sure to leave a little space between each one.

Repeat with Second Sheet: Repeat steps 2-5 with the second sheet of puff pastry and the remaining filling.

Egg Wash: In a small bowl, whisk the egg with a tablespoon of water. Lightly brush the tops of each croissant with the egg wash. This will give them a beautiful golden color as they bake.

Sprinkle with Sugar: If using, lightly sprinkle some sugar on the croissants.

Bake: Bake in a preheated oven at 375°F (190°C) for 18-22 minutes, or until they are puffed up and golden brown. Keep an eye on them – ovens can vary, so you might need to adjust the time slightly.

Cool: Let the croissants cool on the baking sheet for a few minutes before transferring them to a wire rack to cool completely. They're best enjoyed warm, but they're also delicious at room temperature.