Lay out the defrosted puff pastery on a lightly floured board. Invert an oven safe skillet on to the pastry and using the edge of the pan as a guide, cut excess pastry. Remove the pan, and using a fork, prick the circle of pasrty every 1/2 ". Refrigerate until ready to use.
In the skillet, melt the butter over high heat. Add the onions, salt, and thyme. Cook for 2 minutes, stirring constantly. Reduce heat to low and cook for 20 minutes, stirring occasionally. Mix in balsamic vinegar, remove from heat.
When onions are cool, mix in blue cheese. Cover the onion mixture with the puff pasrty disc, pressing it down and tucking in the edges. Bake in a 375F degree oven for about 35 minutes or until golden brown. Remove from the oven and invert it onto a serving platter.
The tart can cool for 10 minutes and served or may be served at room temperature.