Preheat the oven to 375˚ F. Melt the butter in a 12-inch cast-iron skillet over medium heat. Add the onions, thyme, 1 teaspoon salt and a few grinds of pepper. Cook, stirring occasionally, until the onions are very soft and a deep golden brown, 10 to 15 minutes. Stir in the vinegar and cook, scraping up any browned bits, until evaporated, 1 to 2 minutes. Use a slotted spoon to remove the onions to a paper towel–lined plate and wipe the skillet clean.
On the wide end of each dough triangle, spoon a scant tablespoonful of caramelized onions and sprinkle with about 1 tablespoon Swiss cheese. Starting at the wide end, gently roll up the dough around the filling; place in the skillet. There will be some space between each roll, but the rolls will puff up and come together when baked.
Transfer the skillet to the oven and bake until the rolls are golden and puffy, 25 to 30 minutes. Let cool slightly before serving.