Peel and cut the onions into wedges from the roots to the stems.
Melt the butter in an 8-inch oven-safe skillet over a medium heat.
Add the onions and fry for 5 minutes, using tongs to turn them occasionally, until the onions begin to brown slightly.
Add the maple syrup and vinegar to the skillet of onions and cook for a further 5 minutes.
Remove from the heat and arrange the onions in a pattern, then add the sprigs of thyme and season with salt and pepper.
Lightly flour the counter and roll the pastry out so that its diameter is a little larger than the skillet.
Place the pastry onto the skillet and tuck the edges down into the skillet. Poke two holes in the pastry with a knife to allow steam to escape.
Place the skillet into the preheated oven and bake for 20 minutes or until the pastry is golden. Remove from the oven and allow to cool for 5 minutes.
Notes
While the onions are caramelizing, you may need to add a tablespoon of water or two to prevent them from burning.Make sure you cut all the onion wedges all roughly the same size to ensure they cook evenly together.Use flour with the puff pastry to ensure everything stays together.Let the tart cool for 5 minutes to allow the puff pastry to set to prevent it from baking.Be sure to use a skillet that can transfer from the stovetop to the oven properly.