In a pan, combine the onions, maple syrup and thyme. Season with salt and pepper. Bring to a boil, then reduce the heat to medium and simmer for 15 to 18, stirring occasionally, until the onions are caramelized. Remove from heat and let cool.
On a floured work surface, roll out the puff pastry into a 30 cm (12 in) square.
Spread the caramelized onions over the puff pastry. Garnish with shredded duck, brie and basil.
Roll the dough, tightening as you go, to form a cylinder, then place the roll on a baking sheet lined with parchment paper. Place in the freezer for 30 minutes.
When ready to cook, preheat the oven to 190°C (375°F).
Slice the roll into 12 slices and return to the baking sheet, leaving space between the slices.
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