- In a large sauté pan over medium heat add the oil then onions and thyme. Cook the onions, stirring often until a deep golden brown colour, about 25 minutes (reduce heat to medium low if onions start to brown after only 5 minutes). Once the onions start to colour and golden bits stick to the bottom of the pan, stir in the apple – the moisture will pull up the tasty caramelized bits from the pan. Season to taste and cool the onions to room temperature.
- Preheat oven to 400 °F (200 °C) and line a baking tray with parchment paper. Unroll thawed puff pastry, place on the prepared tray and dock pastry with a fork. Spread the cooled onion mixture over the pastry, right to the edges. Bake tart for 20 to 25 minutes, until the bottom of the pastry is a rich golden brown colour. Crumble goat cheese over onions 5 minutes before the tart is done, heating just to melt the cheese slightly.
- Serve the tart warm. Cut into small squares to serve as an appetizer, or cut into 4 to serve as an entrée alongside a tossed salad.
The tart can be made up to a day ahead and reheated in a 325 °F (160 °C) for about 12 minutes.