Pissaladiere - sort-of!

Helping with the nibbles for my friend's party was such fun this weekend.  She is a veggie, and a lover of onions (borderline obsessive - sorry, Anu, but it's true!), so I wanted to give her pissaladiere, but without the anchovies.  And without the traditional bread crust.  Oh, and without the black olives, as there was tapenade to eat too.  So not really pissaladiere, then but just an onion tart!  Anyway, the key is the onions - 3 big red ones and 2 Spanish onions, all sliced thinly, and gently, gently cooked for an hour in 2 tablespoons of olive oil and a knob of butter, with 2 tablespoons of balsamic vinegar, a dessertspoon of brown sugar, a good dessertspoon of fresh thyme leaves and salt and pepper.  It is important to cover the pan and take your time.  Let the onions collapse and slowly caramelise for the right texture and intensity of flavour.  Allow to cool, and then spread over a sheet of good butter puff pastry, leaving a small border.  Bake for 25 minutes or so until the edges have risen, the onions have darkened even more and all is crispy.  Will serve four people, or can be cut into smaller squares as a canape.