Defrost the puff pastry. Heat the oil in a pan, add the thinly sliced onions, sprinkle generously with kosher salt, and cook them on low, stirring intermittently, until the onions are nicely browned and caramelized (this can take a good 45 minutes, do not rush it). While the onions are caramelizing, roll out the defrosted puff pastry on a lightly floured surface. You want it to be about 9 1/2 x 11. Spread the puff with the caramelized onions (but taste the onions first to make sure they are properly salted)--you want the surface covered, but you might have extra; use your discretion. Then lightly dot the onions with the cheese, you still want some onions peaking through, but this is naturally to taste. (Note, I often use Cabot Clothbound--broken into small chunks or shredded ). Sprinkle with thyme, if using. Starting from the short end, fold each side of the dough halfway to the center, then again halfway to the center so that both sides are nearly touching. Finally, fold one side over the other. Refrigerate the roll, covered in plastic wrap for at least 40 minutes. Repeat with the second piece of puff. Before you begin slicing, preheat the oven to 400 F. Slice each roll into 1/4 inch pieces and place the pieces (face up) on parchment covered baking sheets. Bake for 12-15 minutes until golden brown.