Caramelize onions by heating 1 tablespoon butter in a large skillet over medium heat, adding sliced onions with a pinch of salt, and cooking for 25-30 minutes until deep golden brown. Set aside to cool completely.

Mix softened butter with minced garlic, chopped thyme, salt, and pepper until well combined.

Roll out thawed puff pastry on a lightly floured surface into a 12x10 inch rectangle with even thickness.

Spread herb butter evenly over the pastry, leaving a 1/2-inch border on one long edge.

Distribute cooled caramelized onions evenly over the herb butter, then sprinkle crumbled blue cheese on top.

Starting from the long edge opposite the border, tightly roll the pastry into a log and seal the edge by pressing gently.

Wrap the log in plastic wrap and refrigerate for at least 30 minutes to firm up.

Preheat oven to 400°F (200°C) and line baking sheets with parchment paper.

Slice the chilled log into 1/2-inch thick rounds using a sharp knife and arrange on prepared baking sheets with space between each.

Brush tops with beaten egg wash and sprinkle with additional blue cheese if desired.

Bake for 18-22 minutes until puffed, golden brown, and crispy on the edges.

Cool on wire racks for 5 minutes before serving, garnished with fresh thyme sprigs.