Put the onions, butter and oil in a pan and cook over a low heat until the onions are really soft and turning golden – about 20 minutes.
Scoop half of the onions into a small food processor (or use a hand blender) with half the watercress and 3 tbsp of the hazelnuts. Whizz to a pesto texture.
Unroll the pastry sheet, and use a 10-12cm cutter (or similar) to cut 6 circles from the pastry. Space on 1-2 baking sheets, and spread the watercress pesto over the tarts. Scatter over the rest of the caramelised onions, then sit a goat’s cheese slice on each. Finally scatter over the remaining chopped hazelnuts and pop in the fridge to chill for 20-30 minutes – or up to a day if you’re preparing ahead.
Preheat the oven to 200°C (180°C fan oven) Gas Mark 6. Bake for 18-20 minutes until the pastry is puffed and golden, and the cheese oozes.
Lift the tarts from the tin onto serving plates and add a handful more of watercress to each to serve.