For the onions, heat the oil in a non-stick pan. Add the onions, stir well, then cover and cook for 15 mins, stirring every now and then. Stir really well and add the sugar, lots of seasoning and the walnuts, olives and rosemary. Cook for 5 mins more, until soft and very golden.
Heat oven to 200C/180C fan/gas 6. Thinly roll out the pastry, then stamp out 8 fluted rounds with a 10cm cutter. Prick with a fork, then bake for 10 mins. Mix the dressing ingredients together. This can all be prepared a few days ahead.
Pile the onions onto the pastry rounds, top with goat‘s cheese and place a walnut half on top of the cheese. Bake for 10 mins until the cheese is melty. Toss the watercress with the dressing, pile onto plates and serve the pastry on top.