Preheat the oven to 180℃ (170℃ fan)/350°F/gas mark 4. Place the pecans in a bowl and sprinkle with the balsamic vinegar. Sprinkle with the sugar and toss together until the pecans are well coated in the sugar.
Turn the pecans out onto a baking sheet lined with baking parchment and spread out into an even single layer. Bake for 15 minutes stirring the nuts every 5 minutes until golden and the sugar begins to caramelise on the nuts.
Allow to cool before serving. Store in an airtight container in a cool, dry place.