1. Preheat the oven to 350ºF (180º).
2. Melt the butter in a 9-inch (23cm) cast iron skillet. Cut the Belgian endive spears in half lengthwise. Sprinkle some salt and pepper over the melted butter in the pan and lay the endive spears, cut side down, in the bottom of the pan. They may not all seem to fit right now, but really try to crowd them in together, so they’re as close as possible.
3. Cook the endive spears over medium-high heat, pressing them down as they cook, but doing your best not to disturb or move them around, so they brown nicely on the underside. Once the cut sides of the endive are well-browned about 4 minutes, sprinkle the endives with the sugar or drizzle with honey, cover the pan and put in the oven to bake until the endive spears are almost cooked through, about 25 minutes, depending on their size.
4. Remove the pan of endive from the oven. Distribute the blue cheese cubes in the spaces between the endives, as well as on top of them.
5. On a lightly floured surface, roll the puff pastry to a 12-inch (31cm) circle. Drape the dough over the endives, and tuck the outside edges of the dough between the endives and the inside of the pan.
6. Bake the tart until the crust is deep golden brown, about 25 minutes. Remove from the oven and overturn a serving platter on top. Holding both the pan and the plate, wearing oven mitts (being careful since some very hot liquid may drip out of the pan when doing this), turn both the plate and pan over to release the tart from the pan. Reunite any endive spears that may have stuck to the pan and bits of cheese. If you’d like, add a few more cubes of blue cheese over the top, while the tart is still warm.